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May  24— 2M. 


Hughes' 
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ON  Diseases  of  the  Skin. 


By  DANIEL  E.  HUGHES,  M.D.,  Late  Chief  Resident  Physician, 
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Jefferson  Medical  College.  Edited  by  R.  J.  E.  SCOTT,  M.A.,  B.C.L., 
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63  Illustrations,    12mo.        xxiv  -|-  810  Pages,    Cloth,  SJ^.OO  postpaid. 

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BRUBAKER.    PHYSIOLOGY.    Fifteenth  Edition,  with  26  Illustrations.    Enlarged 

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LANDIS.    OBSTETRICS.    Ninth    Edition.     Revised    and    Edited    by    Wm.    H. 

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WRITING.     Eighth  Revised  Edition. 
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Urinalysis,  Animal  Chemistry,  Chemistry  of  Milk,  Blood,  Tissues,  the  Secretions, 

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Third  Edition.     With  59  Illustrations. 


BLAKISTON'S       CONIF'ENDS 

A    COMPEND 

OF 

HUMAN    PHYSIOLOGY 

ESPECIALLY  ADAPTED  FOR  THE  USE 
OF  MEDICAL  STUDENTS 


BY 
ALBERT  P.  BRUBAKER,  A.M.,  M.D. 

AUTHOR    OF    "A    TEXT-BOOK    OF^PHYSIOLOGY;"    PROFESSOR  OF  PHYSIOLOGY  AND 

MEDICAL  JURISPRUDENCE  IN  THE  JEFFERSON  MEDICAL  COLLEGE;    FORMERLY 

PROFESSOR  OF  PHYSIOLOGY  IN  THE    PENNSVLV/ NIA    COLLEGE  OF  DENTAL 

SURGERY;    FORMERLY  LECTURER    CK    AWATOriY    .\NE     ^HYSIOLuHY  11^ 

THE  DREXEL  INSTITUTE  OF  ART,  SCifi-NOE,  AND  INDUSTRY;  FELLOW 

OF  THE  COLLEGE  OF  PHYSICIANS  OF  PHILADELPii]^   '      '       ,  j  '       '    ', 


FIFTEENTH   EDITION 
WITH  26  ILLUSTRATIONS 


PHILADELPHIA 
BLAKISTON'S   SON   &   CO. 

1012  WALNUT  STREET 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/compendphysiologyOObrubrich 


CONTENTS 


Page 

Introduction i 

Physiology  of  the  Cell S 

Physiology  of  the  Skeleton 9 

General  Physiology  of  Muscular  Tissue 14 

Special  Physiology  of  Muscles 28 

Physiology  of  Nerve  Tissue ^ 34 

Foods  and  Dietetics 52 

Digestion 61 

Absorption 86 

Blood 94 

Circulation  of  the  Blood 102 

Respiration 123 

Animal  Heat < 136 

Excretion 140 

External  Secretions 152 

Internal  Secretions 162 

The  Central  Organs  of  the  Nerve  System 170 

Spinal  Cord 172 

Medulla  Oblongata 184 

Pons  Varolii ,    ...  185 

Crura  Cerebri 186 

Corpora  Quadrigemina 187 

Corpora  Striata  and  Optic  Thalami i88 

Cerebrum 191 

Cerebellum 203 

Autonomic  Nerve  System 205 

Cranial  Nerves 209 

Sense  of  Touch 222 

Sense  of  Taste 224 

Sense  of  Smell 225 

Sense  of  Sight 226 

vii 


VIU  CONTENTS 

Page 

Sense  of  Hearing ^37 

Phonation  and  Articulate  Speech 246 

Reproduction 251 

Index 261 


A  COMPEND 


OF 


HUMAN  PHYSIOLOGY 


Introduction. — An  animal  organism  in  the  living  condition  exhibits  a 
series  of  phenomena  which  relate  to  growth,  movement,  mentality,  and  re- 
production. During  the  period  preceding  birth,  as  well  as  during  the 
period  included  between  birth  and  adult  life,  the  individual  grows  in  size 
and  complexity  from  the  introduction  and  assimilation  of  material  from 
without.  Throughout  its  life  the  animal  exhibits  a  series  of  movements, 
in  virtue  of  which  it  not  only  changes  the  relation  of  one  part  of  its  body 
to  another,  but  also  changes  its  position  in  space.  If,  in  the  execution  of 
these  movements,  the  parts  are  directed  to  the  overcoming  of  opposing 
forces,  such  as  gravity,  friction,  cohesion,  elasticity,  etc.,  the  animal  may 
be  said  to  be  doing  work.  The  result  of  normal  growth  is  the  attainment 
of  a  physical  development  that  will  enable  the  animal,  and,  more  espe- 
cially, man,  to  perform  the  work  necessitated  by  the  nature  of  its  environ- 
ment and  the  character  of  its  organization.  In  man,  and  probably  in 
lower  animals  as  well,  mentality  manifests  itself  as  intellect,  feeling,  and 
volition.  At  a  definite  period  in  the  life  of  the  animal  it  reproduces  itself, 
in  consequence  of  which  the  species  to  which  it  belongs  is  perpetuated. 

The  study  of  the  phenomena  of  growth,  movement,  mentality,  and  re- 
production constitutes  the  science  of  Animal  Physiology.  But  as  these 
general  activities  are  the  resultant  of  and  dependent  on  the  special  activi- 
ties of  the  individual  structures  of  which  an  animal  body  is  composed, 
Physiology  in  its  more  restricted  and  generally  accepted  sense  is  the 
science  which  investigates  the  actions  or  functions  of  the  individual 
organs  and  tissues  of  the  body  and  the  physical  and  chemic  conditions 
which  underlie  and  determine  them. 


2  HUMAN   PHYSIOLOGY 

This  may  naturally  be  divided  into: 

1.  Individual  physiology y  the  object  of  which  is  a  study  of  the  vital 
phenomena  or  functions  exhibited  by  the  organs  of  any  individual  animal. 

2.  Comparative  physiology,  the  object  of  which  is  a  comparison  of  the 
vital  phenomena  or  functions  exhibited  by  the  organs  of  two  or  more  ani- 
mals, with  a  view  of  unfolding  their  points  of  resemblance  or  dissimilarity. 

Human  physiology  is  that  department  of  physiologic  science  which  has 
for  its  object  the  study  of  the  functions  of  the  organs  of  the  human  body  in 
a  state  of  health. 

If  the  body  of  any  animal  be  dissected,  it  will  be  found  to  be  composed  of 
a  number  of  well-defined  structures,  such  as  heart,  lungs,  stomach,  brain, 
eye,  etc.,  to  which  the  term  organ  was  originally  applied,  for  the  reason 
that  they  were  supposed  to  be  instruments  capable  of  performing  some 
important  act  or  function  in  the  general  activities  of  the  body.  Though 
the  term  organ  is  usually  employed  to  designate  the  larger  and  more 
familiar  structures  just  mentioned,  it  is  equally  applicable  to  a  large 
number  of  other  structures  which,  though  possibly  less  obvious,  are 
equally  important  in  maintaining  the  life  of  the  individual — e.g.,  bones, 
muscles,  nerves,  skin,  teeth,  glands,  blood-vessels,  etc.  Indeed,  any  com- 
plexly organized  structure  capable  of  performing  some  function  may  be 
described  as  an  organ.  A  description  of  the  various  organs  which  make 
up  the  body  of  an  animal,  their  external  form,  their  internal  arrangement, 
their  relations  to  one  another,  constitutes  the  science  of  Animal  Anatomy. 

This  may  naturally  be  divided  into : 

1.  Individual  anatomy,  the  object  of  which  is  the  investigation  of  the 
construction,  form,  and  arrangement  of  the  organs  of  any  individual 
animal. 

2.  Comparative  anatomy,  the  object  of  which  is  a  comparison  of  the 
organs  of  two  or  more  animals,  with  a  view  of  determining  their  points  of 
resemblance  or  dissimilarity. 

If  the  organs,  however,  are  subjected  to  a  further  analysis,  they  can  be 
resolved  into  simple  structures,  apparently  homogeneous,  to  which  the 
name  tissue  has  been  given — e.g.,  epithelial,  connective,  muscle,  and  nerve 
tissue.  When  the  tissues  are  subjected  to  microscopic  analysis,  it  is 
found  that  they  are  not  homogeneous  in  structure,  but  composed  of  still 
simpler  elements,  termed  cells  and  fibers.  The  investigation  of  the  inter- 
nal structure  of  the  organs,  the  physical  properties  and  structure  of  the 
tissues,  as  well  as  the  structure  of  their  component  elements,  the  cells  and 
fibers,  constitutes  a  department  of  anatomic  science  known  as  Histology, 
or  as  it  is  prosecuted  largely  with  the  microscope,  Microscopic  Anatomy. 


PHYSIOLOGIC  APPARATUS  3 

Human  anatomy  is  the  department  of  anatomic  science  which  has  for  its 
object  the  investigation  of  the  construction  of  the  human  body. 

Inasmuch  as  the  study  of  function  or  physiology  is  associated  with  and 
dependent  on  a  knowledge  of  structure,  it  is  essential  that  the  student 
should  have  a  general  acquaintance  with  the  anatomy  and  histology  not 
only  of  man  but  of  typical  forms  of  lower  animal  life  as  well.  Moreover, 
since  it  has  been  demonstrated  that  every  exhibition  of  functional  activity 
is  associated  with  changes  in  the  composition  of  the  structures,  It  has  been 
apparent  that  a  knowledge  of  the  chemical  composition  of  the  body,  as 
well  as  the  successive  changes  in  composition  which  it  undergoes  when  in 
a  state  of  functional  activity,  is  necessary  to  a  correct  understanding  of 
the  intimate  nature  of  physiologic  processes. 

Anatomic  Systems. — All  the  organs  of  the  body  which  have  certain 
peculiarities  of  structure  in  common  are  classified  by  anatomists  into  sys- 
tems— e.g.,  the  bones,  collectively,  constitute  the  bony  or  osseous  sys- 
tem; the  muscles,  the  nerves,  the  skin,  constitute,  respectively,  the 
muscular,  the  nervous,  and  the  tegumentary  system. 

Physiologic  Apparatus. — More  important  from  a  physiologic  point  of 
view  than  a  classification  of  organs  based  on  similarities  of  structure  is  the 
natural  association  of  two  or  more  organs  acting  together  for  the  accom- 
plishment of  some  definite  object,  and  to  which  the  term  physiologic 
apparatus  has  been  applied.  While  in  the  community  of  organs  which 
together  constitute  the  animal  body  each  one  performs  some  definite  func- 
tion, and  the  harmonious  cooperation  of  all  is  necessary  to  the  life  of  the 
individual,  everywhere  it  is  found  that  two  or  more  organs,  though  perform- 
ing totally  distinct  functions,  are  cooperating  for  the  accomplishment  of 
some  larger  or  compound  function  in  which  their  individual  functions  are 
blended — e.g.,  the  mouth,  stomach,  and  intestines,  with  the  glands  con- 
nected with  them,  constitute  the  digestive  apparatus,  the  object  or  function 
of  which  is  the  complete  digestion  of  the  food.  The  capillary  blood- 
vessels and  lymphatic  vessels  of  the  body,  and  especially  those  in  relation 
to  the  villi  of  the  small  intestine,  constitute  the  absorptive  apparatus,  the 
function  of  which  is  the  introduction  of  new  material  into  the  blood.  The 
heart  and  blood-vessels  constitute  the  circulatory  apparatus,  the  function 
of  which  is  the  distribution  of  blood  to  all  portions  of  the  body.  The 
lungs  and  trachea,  together  with  the  diaphragm  and  the  walls  of  the  chest, 
constitute  the  respiratory  apparatus,  the  function  of  which  is  the  intro- 
duction of  oxygen  into  the  blood  and  the  elimination  from  it  of  carbon 
dioxid  and  other  injurious  products.  The  kidneys,  the  ureters,  and  the 
bladder  constitute  the  urinary  apparatus.     The  skin,  with  its  sweat- 


4  HUMAN  PHYSIOLOGY 

glands,  constitutes  the  perspiratory  apparatus,  the  functions  of  both  being 
the  excretion  of  waste  products  from  the  body.  The  liver,  the  pancreas 
the  mammary  glands,  as  well  as  other  glands,  each  form  a  secretory  appa- 
ratus which  elaborates  some  specific  material  necessary  to  the  nutrition 
of  the  individual.  The  functions  of  these  different  physiologic  apparatus 
— e.g.,  digestion,  absorption  of  food,  elaboration  of  blood,  circulation  of 
blood,  respiration,  production  of  heat,  secretion  and  excretion — are 
classified  as  nutritive  functions,  and  have  for  their  final  object  the  preser- 
vation of  the  individual. 

The  nerves  and  muscles  constitute  the  nervo-muscular  apparatus,  the 
function  of  which  is  the  production  of  motion.  The  eye,  the  ear,  the  nose, 
the  tongue,  and  the  skin,  with  their  related  structures,  constitute,  respec- 
tively, the  visual,  auditory,  olfactory,  gustatory,  and  tactile  apparatus,  the 
function  of  which,  as  a  whole,  is  the  reception  of  impressions  and  the  trans- 
mission of  nerve  impulses  to  the  brain,  where  they  give  rise  to  visual, 
auditory,  olfactory,  gustatory,  and  tactile  sensations. 

The  brain,  in  association  with  the  sense  organs,  forms  an  apparatus 
related  to  mental  processes.  The  larynx  and  its  accessory  organs — the 
lungs,  trachea,  respiratory  muscles,  the  mouth  and  resonant  cavities  of 
the  face — form  the  vocal  and  articulating  apparatus,  by  means  of  which 
voice  and  articulate  speech  are  produced.  The  functions  exhibited  by 
the  apparatus  just  mentioned — viz.,  motion,  sensation,  language,  mental 
and  moral  manifestations — are  classified  as  functions  of  relation,  as  they 
serve  to  bring  the  individual  into  conscious  relationship  with  the  external 
world. 

The  ovaries  and  the  testes  are  the  essential  reproductive  organs,  the 
former  producing  the  germ-cell,  the  latter  the  spermatic  element;  together 
with  their  related  structures — the  fallopian  tubes,  uterus,  and  vagina  in  the 
female,  and  the  urogenital  canal  in  the  male — they  constitute  the  repro- 
ductive apparatus  characteristic  of  the  two  sexes.  Their  cooperation 
results  in  the  union  of  the  germ-cell  and  spermatic  element  and  the  conse- 
quent development  of  a  new  being.  The  function  of  reproduction  serves 
to  perpetuate  the  species  to  which  the  individual  belongs. 

The  animal  body  is  therefore  not  a  homogeneous  organism,  but  one 
composed  of  a  large  number  of  widely  dissimilar  but  related  organs.  But 
as  all  vertebrate  animals  have  the  same  general  plan  of  organization,  there 
is  a  marked  similarity  both  in  form  and  structure  among  corresponding 
parts  of  different  animals.  Hence  it  is  that  in  the  study  of  human  anat- 
omy a  knowledge  of  the  form,  construction,  and  arrangement  of  the  organs 
in  different  types  of  animal  life  is  essential  to  its  correct  interpretation;  also 
it  is  that  in  the  investigation  and  comprehension  of  the  complex  problems 


PHYSIOLOGY   OF   THE   CELL  5 

of  human  physiology  a  knowledge  of  the  functions  of  the  organs  as  they 
manifest  themselves  in  the  different  types  of  animal  life  is  indispensable. 
As  many  of  the  functions  of  the  human  body  are  not  only  complex,  but 
the  organs  exhibiting  them  are  practically  inaccessible  to  investigation,  we 
must  supplement  our  knowledge  and  judge  of  their  functions  by  analogy, 
by  attributing  to  them,  within  certain  limits,  the  functions  revealed  by 
experimentation  upon  the  corresponding  but  simpler  organs  of  lower  ani- 
mals. This  experimental  knowledge  corrected  by  a  study  of  the  clinical 
phenomena  of  disease  and  the  results  of  post-mortem  investigations,  forms 
the  basis  of  modern  human  physiology. 

PHYSIOLOGY  OF  THE  CELL 

A  histologic  analysis  of  the  tissues  shows  that  they  can  be  resolved  into 
simpler  elements,  termed  cells,  which  may,  therefore,  be  regarded  as  the 
primary  units  of  structure.  Though  cells  vary  considerably  in  shape,  size 
and  chemic  composition  in  the  different  tissues  of  the  adult  body,  they  are 
nevertheless,  descendants  from  typical  cells,  known  as  embryonic  or 
undifferentiated  cells,  the  first  offspring  of  the  fertilized  ovum.  Ascend- 
ing the  line  of  embryonic  development,  it  will  be  found  that  every  organ- 
ized body  originates  in  a  single  cell — the  ovum.  As  the  cell  is  the 
elementary  unit  of  all  tissues,  the  function  of  each  tissue  must  be  referred 
to  the  function  of  the  cell.  Hence  the  cell  may  be  defined  as  the  primary 
anatomic  and  physiologic  unit  of  the  organic  world,  to  which  every  exhibi- 
tion of  life,  whether  normal  or  abnormal  is  to  be  referred. 

Structure  of  Cells. — Though  cells  vary  in  shape  and  size  and  internal 
structure  in  different  portions  of  the  body,  a  typical  cell  may  be  said  to 
consist  mainly  of  a  gelatinous  substance  forming  the  body  of  the  cell, 
termed  cytoplasm  or  bioplasm,  in  which  is  embedded  a  smaller  spheric  body, 
the  nucleus.  Within  the  nucleus  there  is  frequently  a  still  smaller  body 
the  nucleolus.  The  shape  of  the  adult  cell  varies  according  to  the  tissue 
in  which  it  is  found ;  when  young  and  free  to  move  in  a  fluid  medium,  the 
cell  assumes  a  spheric  form,  but  when  subjected  to  pressure,  may  become 
cylindric,  fusiform,  polygonal,  or  stellate.  Cells  vary  in  size  within  wide 
limits,  ranging  from  J^  200  of  an  inch,  the  diameter  of  a  red  blood-corpuscle 
to  J^  00  of  an  inch,  the  diameter  of  the  large  cells  in  the  gray  matter  of  the 
spinal  cord. 

The  cell  cytoplasm  consists  of  a  soft,  semifluid,  gelatinous  material,  vary- 
ing somewhat  in  appearance  in  different  tissues.  Though  frequently 
homogeneous,  it  often  exhibits  a  finely  granular  appearance  under  medium 


6  HUMAN  PHYSIOLOGY 

powers  of  the  microscope.  Young  cells  consists  almost  entirely  of  clear 
cytoplasm,  mature  cells  contain,  according  to  the  tissue  in  which  they  are 
found,  material  of  an  entirely  different  character — e.g.,  small  globules  of 
fat,  granules  of  glycogen,  mucigen,  pigments,  digestive  ferments,  etc. 
Under  high  powers  of  the  microscope  the  cytoplasm  is  found  to  be  per- 
vaded by  a  network  of  fibers,  termed  spongioplasm,  in  the  meshes  of  which 
is  contained  a  clearer  and  more  fluent  substance,  the  hyaloplasm.  The 
relative  amount  of  these  two  constituents  varies  in  different  cells,  the 
proportion  of  hyaloplasm  being  usually  greater  in  young  cells.  The 
arrangement  of  the  fibers  forming  the  spongioplasm  also  varies,  the  fibers 
having  sometimes  a  radial  direction,  in  others  a  concentric  disposition, 
but  most  frequently  being  distributed  evenly  in  all  directions.  In  many 
cells  the  outer  portion  of  the  cell  cytoplasm  undergoes  chemic  changes 
and  is  transformed  into  a  thin,  transparent,  homogenous  membrane — the 
cell  membrane — which  completely  incloses  the  cell  substance.  The  cell 
membrane  is  permeable  to  water  and  watery  solutions  of  various  inorganic 
and  organic  substances.    It  is,  however,  not  an  essential  part  of  the  cell. 

The  nucleus  is  a  small  vesicular  body  embedded  in  the  cytoplasm  near 
the  center  of  the  cell.  In  the  resting  condition  of  the  cell  it  consists  of  a 
distinct  membrane,  composed  of  amphipyrenin,  inclosing  the  nuclear 
contents.  The  latter  consists  of  a  homogenous  amorphous  substance — the 
nuclear  matrix — in  which  is  embedded  the  nuclear  network.  It  can  often 
be  seen  that  a  portion  of  one  side  of  the  nucleus,  called  the  pole,  is  free 
from  this  network.  The  main  cords  of  the  network  are  arranged  as  V- 
shaped  loops  about  it.  These  main  cords  send  out  secondary  branches  or 
twigs,  which,  uniting  with  one  another,  complete  the  network.  The  nu- 
clear cords  are  composed  of  granules  of  chromatin — so  called  because  of  its 
affinity  for  certain  staining  materials — held  together  by  an  achromatin 
substance  known  as  linen.  Besides  the  nuclear  network,  there  are  em- 
bedded in  the  nuclear  matrix  one  or  more  small  bodies  composed  of  pyrenin 
known  as  nucleoli.  At  the  pole  of  the  nucleus,  either  within  or  just  with- 
out in  the  cytoplasm,  is  a  small  body,  the  centrosome,  or  pole  corpuscle. 

Chemic  Composition  of  the  Cell. — The  composition  of  living  protoplasm 
is  difficult  of  determination,  for  the  reason  that  all  chemic  and  physical 
methods  employed  for  its  analysis  destroy  its  vitality,  and  the  products 
obtained  are  peculiar  to  dead  rather  than  to  living  matter.  Moreover, 
as  protoplasm  is  the  seat  of  constructive  and  destructive  processes,  it  is  not 
easy  to  determine  whether  the  products  of  analysis  are  crude  food  constitu- 
ents or  cleavage  or  disintegration  products.  Nevertheless,  chemic  inves- 
tigations have  shown  that  even  in  the  living  condition  protoplasm  is  a 


PHYSIOLOGY   OF   THE  CELL  7 

highly  complex  compound — the  resultant  of  the  intimate  union  of  many 
different  substances.  About  seventy-five  per  cent,  of  protoplasm  consists 
of  water  and  twenty-five  per  cent,  of  solids,  of  which  the  more  important 
compounds  are  various  nucleo-proteins  (characterized  by  their  large 
percentage  of  phosphorus),  globulins,  traces  of  lecithin,  cholesterin  and 
frequently  fat  and  carbohydrates.  Inorganic  salts,  especially  the  potas- 
sium, sodium,  and  calcium  chlorids  and  phosphates,  are  almost  invariable 
and  essential  constituents. 

MANIFESTATIONS  OF  CELL  LIFE 

Growth,  the  Maintenance  of  Nutrition,  and  Reproduction. — All  cells 
exhibit  three  fundamental  properties  of  life — viz.,  growth,  the  main- 
tenance of  their  nutrition,  and  reproduction.  Growth  is  an  increase  in 
size.  When  newly  reproduced  all  cells  are  extremely  small,  but  in  conse- 
quence of  their  organization  and  the  character  of  their  surrounding 
medium,  they  gradually  grow  until  they  attain  the  size  characteristic 
of  the  adult  state. 

Nutrition  may  be  defined  as  the  sum  of  the  processes  concerned  in  the 
maintenance  of  the  physiologic  condition  of  the  cell  and  includes  both 
growth  and  repair.  So  long  as  this  is  accomplished,  the  cells  and  the 
tissues  which  are  formed  by  them  continue  to  exhibit  their  functions  or 
their  characteristic  modes  of  activity.  Both  growth  and  nutrition  are 
dependent  on  the  power  which  living  material  possesses  of  not  only 
absorbing  nutritive  material  from  the  surrounding  medium,  the  lymph, 
but  of  subsequently  assimilating  it,  organizing  it,  transforming  it  into 
material  like  itself  and  endowing  it  with  its  own  physiologic  properties. 

In  the  physiologic  condition  the  living  material  of  the  cell,  the  bio- 
plasm, is  the  seat  of  a  series  of  chemic  changes  which  vary  in  degree  from 
moment  to  moment  in  accordance  with  the  degree  of  functional  activity, 
and  on  the  continuance  of  which  all  life  phenomena  depend.  Some  of 
these  chemic  changes  are  related  to  or  connected  with  the  molecules  of 
the  living  material,  while  othess  are  connected  with  the  food  material 
supplied  to  them.  Of  the  chemic  changes  occurring  within  the  mole- 
cules some  are  destructive,  dissimilative  or  disintegrative  in  character, 
whereby  the  molecule  is  in  part  eventually  reduced  through  a  series  of 
descending  chemic  stages  to  simpler  compounds  which,  apparently  of 
no  use  in  the  cell,  are  eliminated  from  it.  It  is,  therefore,  said  that  the 
living  material  undergoes  molecular  disintegration  as  a  result  of  func- 
tional activity.  To  these  changes  the  term  kataholism  is  also  applied. 
Other  of  these  changes  are  constructive,  assimilative  or  integrative  in 


8  HUMAN  PHYSIOLOGY 

character,  whereby  a  part  at  least  of  the  food  material  furnished  by  the 
blood-plasma  is  transformed  through  a  series  of  ascending  chemic  stages 
into  living  material,  and  whereby  it  is  repaired  and  its  former  physio- 
logic condition  restored.  It  is,  therefore,  said  that  the  living  material 
undergoes  molecular  integration  as  a  preparation  for  functional  activity. 
To  these  changes  the  term  anaholism  is  also  applied.  During  the  course 
of  its  physiologic  activities  the  cell  bioplasm  produces  materials  of  an 
entirely  different  character  which  vary  with  the  cell,  such  as  fat,  glycogen, 
mucigen,  pigments,  ferments,  etc.,  which  are  generally  spoken  of  as 
metabolic  products. 

Living  material  has  also  a  temperature  varying  in  degree  in  different 
species  of  animals  as  well  as  in  different  parts  of  the  same  animal.  Here 
as  elsewhere  the  temperature  is  due  to  heat  liberated  from  organic  com- 
pounds through  disruption  and  subsequent  oxidation  to  simpler  com- 
pounds. Though  some  of  the  heat  liberated  may  come  from  the  tissue 
molecules,  the  larger  part  by  far  comes  from  the  food  molecules — sugar, 
fat,  and  protein,  constituents  of  the  fluids  circulating  in  the  tissue  spaces. 
These  foods  carry  into  the  body  potential  energy,  ultimately  derived 
from  the  sun.  Whey  they  are  disrupted  and  oxidized  the  potential 
energy  is  transformed  into  kinetic  energy  which  manifests  itself  for  the 
most  part  as  heat.  To  the  sum  total  of  all  the  chemic  changes  occurring 
in  tissues  and  foods  the  term  metabolism  is  given. 

Physiologic  Properties  of  Protoplasm. — All  living  protoplasm  possesses 
properties  which  serve  to  distinguish  and  characterize  it — viz.,  irritability, 
conductivity,  and  motility. 

Irritability,  or  the  power  of  reacting  in  a  definite  manner  to  some  form 
of  external  excitation,  whether  mechanical,  chemic,  or  electric,  is  a  funda- 
mental property  of  all  living  protoplasm.  The  character  and  extent  of 
the  reaction  will  vary,  and  will  depend  both  on  the  nature  of  the  proto- 
plasm and  the  character  and  strength  of  the  stimulus.  If  the  protoplasm 
be  muscle,  the  response  will  be  a  contraction;  if  it  be  gland,  the  response 
will  be  secretion;  if  it  be  nerve,  the  response  will  be  a  sensation  or  some 
other  form  of  nerve  activity. 

Conductivity^  or  the  power  of  transmitting  moleculat  disturbances 
arising  at  one  point  to  all  portions  of  the  irritable  material,  is  also  a  char- 
acteristic feature  of  all  protoplasm.  This  power,  however,  is  best  de- 
veloped in  that  form  of  protoplasm  found  in  nerves,  which  serves  to 
transmit,  with  extreme  rapidity,  molecular  disturbances  arising  at  the 
periphery  to  the  brain,  as  well  as  in  the  reverse  direction.  Muscle 
protoplasm  also  possesses  the  same  power  in  a  high  degree. 


THE  PHYSIOLOGY  OF  THE  SKELETON  9 

Motility^  or  the  power  of  executing  apparently  spontaneous  move- 
ments, is  exhibited  by  many  forms  of  cell  protoplasm.  In  addition  to 
the  molecular  movements  which  take  place  in  certain  cells,  other  forms 
of  movement  are  exhibited,  more  or  less  constantly,  by  many  cells  in 
the  animal  body— e.^.,  the  waving  of  cilia,  the  ameboid  movements  and 
migrations  of  white  blood  corpuscles,  the  activities  of  spermatozooids, 
the  projections  of  pseudopodia,  etc.  These  movements,  arising  without 
any  recognizable  cause,  are  frequently  spoken  of  as  spontaneous.  Strictly 
speaking,  however,  all  protoplasmic  movement  is  the  resultant  of  natural 
causes,  the  true  nature  of  which  is  beyond  the  reach  of  present  methods 
of  investigation. 

Reproduction. — Cells  reproduce  themselves  in  the  higher  animals  in 
two  ways — by  direct  division  and  by  indirect  division,  or  karyokinesis. 
In  the  former  the  nucleus  becomes  constricted,  and  divides  without  any 
special  grouping  of  the  nuclear  elements.  It  is  probable  that  this  occurs 
only  in  disintegrating  cells,  and  never  in  a  physiologic  multiplication. 
In  division  by  karyokinesis  there  is  a  progressive  rearranging  and  definite 
grouping  of  the  nucleus,  the  result  of  which  changes  is  the  division  of 
the  centrosome,  the  chromatin,  and  the  rest  of  the  nucleus  into  two 
equal  portions,  which  form  the  nuclei.  Following  the  division  of  the 
nuclei,  the  protoplasm  becomes  constricted  midway  between  the  young 
nuclei.  This  constriction  gradually  deepens  until  the  original  cell  is 
divided,  with  the  formation  of  two  complete  cells. 

THE  PHYSIOLOGY  OF  THE  SKELETON 

The  animal  body  is  characterized  by  the  power  of  executing  a  great 
variety  of  movements,  all  of  which  have  reference  to  a  change  of  relation 
of  one  part  of  the  body  to  another,  or  to  a  change  of  position  of  the  indivi- 
dual in  its  environment,  as  in  the  various  acts  of  locomotion.  If  in  the 
execution  of  these  movements  the  different  parts  are  applied  or  directed 
to  the  overcoming  of  opposing  forces  in  the  environment,  the  animal  is  said 
to  be  doing  work.  In  the  conception  of  the  animal  body  as  a  machine 
for  the  accomplishment  of  work  the  skeleton,  the  muscle  and  nerve  tissues, 
constitute  the  three  primary  mechanisms,  all  of  which  bear  certain  defi- 
nite relations  one  to  another. 

The  skeleton  in  its  entirety  determines  the  plan  of  organization  of  the 
animal  body  and  imparts  to  it  its  characteristic  features.  In  its  entirety 
it  serves  for  the  attachment  of  muscles,  the  support  of  viscera  and  by 
reason  of  the  relation  of  the  bones  one  to  another,  permits  of  a  great  variety 


10  HUMAN   PHYSIOLOGY 

of  movements.     The  skeleton  may  be  divided  into  an  axial  and  an  appen- 
dicular portion. 

The  Axial  Portion. — The  axial  portion  consists  of  the  bones  of  the  head, 
of  the  vertebral  column  and  the  ribs.  The  vertebral  column  is  the  founda- 
tion element  and  the  center  around  which  the  appendicular  portions  are 
developed  and  arranged  with  a  certain  degree  of  conformity.  It  is  com- 
posed of  a  series  of  superimposed  bones,  termed  vertebrae,  which  increase  in 
size  from  above  downward  as  far  as  the  brim  of  the  pelvic  cavity.  Supe- 
riorly, it  supports  the  skull;  laterally,  it  affords  attachment  for  the  ribs, 
which  in  turn  support  the  weight  of  the  upper  extremities;  below,  it  rests 
upon  the  pelvic  bones,  which  transmit  the  weight  of  the  body  to  the  in- 
ferior extremities.  The  bodies  of  the  vertebrae  are  united  one  to  another 
by  tough  elastic  discs  of  fibro-cartilage,  which,  collectively,  constitute 
about  one-quarter  of  the  length  of  the  vertebral  column.  The  vertebric  are 
held  together  by  ligaments  situated  on  the  anterior  and  posterior  surfaces 
of  their  bodies,  and  by  short,  elastic  ligaments  between  the  neural  arches 
and  processes.  These  structures  combine  to  render  the  vertebral  column 
elastic  and  flexible,  and  enable  it  to  resist  and  diminish  the  force  of  shocks 
communicated  to  it.  The  character  and  the  arrangement  of  the  bones  of 
the  axial  portion  endow  the  animal  mechanism  with  a  certain  degree  of 
fixity  combined  with  slight  mobility. 

The  Appendicular  Portion. — The  appendicular  portion  consists  of  the 
bones  of  the  arms  and  legs,  the  scapular  and  pelvic  arches.  By  reason  of 
its  character  and  anatomic  arrangement,  the  animal  body  is  endowed 
with  extreme  mobility,  enabling  the  animal  to  execute  a  great  variety  of 
rapid  and  extensive  movements  which,  however,  vary  in  degree  in  different 
animals  in  accordance  with  their  organization  and  the  nature  of  their 
environment. 

For  the  manifestation  of  the  activities  of  the  animal  it  is  essential  that 
the  relation  of  the  various  portions  of  the  bony  skeleton  to  one  another 
shall  be  such  as  to  permit  of  movement  while  yet  retaining  close  apposi- 
tion. This  is  accomplished  by  the  mechanical  conditions  which  have  been 
evolved  at  the  points  of  union  of  bones,  and  which  are  technically  known 
as  articulations  or  joints. 

A  consideration  of  the  body  movements  involves  an  account  of  (i)  the 
static  conditions,  or  those  states  of  equilibrium  in  which  the  body  is  at 
rest — e.g.,  standing,  sitting;  (2)  the  dynamic  conditions  or  those  states  of 
activity  characterized  by  movement — e.g.j  walking,  running,  etc.  In  this 
connection,  however,  only  those  physical  and  physiologic  peculiarities  of 


THE  PHYSIOLOGY  OF  THE  SKELETON  II 

the  skeleton,  especially  in  its  relation  to  joints,  will  be  referred  to  which 
underlie  and  determine  both  the  static  and  dynamic  states  of  the  body. 

Structure  of  Joints. — The  structures  entering  into  the  formation  of 
joints  are : 

1.  Bones,  the  articulating  surfaces  of  which  are  often  more  or  less  ex- 
panded, especially  in  the  case  of  long  bones,  and  at  the  same  time  variously 
modified  and  adapted  to  one  another  in  accordance  with  the  character  and 
extent  of  the  movements  which  there  take  place. 

2.  Hyaline  cartilage,  which  is  closely  applied  to  the  articulating  end  of 
each  bone.  The  smoothness  of  this  form  of  cartilage  facilitates  the  move- 
ments of  the  opposing  surfaces,  while  its  elasticity  diminishes  the  force  of 
shocks  and  jars  imparted  to  the  bones  during  various  muscular  acts.  In 
a  number  of  joints,  plates  or  discs  of  white  fibro-cartilage  are  inserted 
between  the  surfaces  of  the  bones. 

3.  A  synovial  membrane,  which  is  attached  to  the  edge  of  the  hyaline 
cartilage  entirely  inclosing  the  cavity  of  the  joint.  This  membrane  is 
composed  largely  of  connective  tissue,  the  inner  surface  of  which  is  lined 
by  endothelial  cells,  which  secrete  a  clear,  colorless,  ^viscid  fluid — the 
synovia.  This  fluid  not  only  fills  up  the  joint-cavity,  but,  flowing  over  the 
articulating  surfaces,  diminishes  or  prevents  friction. 

4.  Ligaments — tough,  inelastic  bands,  composed  of  white  fibrous  tissue 
— which  pass  from  bone  to  bone  in  various  directions  on  the  different 
aspects  of  the  joint.  As  white  fibrous  tissue  is  inextensible  but  pliant, 
ligaments  assist  in  keeping  the  bones  in  apposition,  and  prevent  displace- 
ment while  yet  permitting  of  free  and  easy  movements. 

Classification  of  Joints. — All  joints  may  be  divided,  according  to  the 
extent  and  kind  of  movements  permitted  by  them,  into  (i)  diarthroses; 
(2)  amphiarthroses;  (3)  synarthroses. 

A.  Diarthroses. — In  this  division  of  the  joints  are  included  all  those 
which  permit  of  free  movement.  In  the  majority  of  instances  the  articu- 
lating surfaces  are  mutually  adapted  to  each  other.  If  the  articulating 
surface  of  one  bone  is  convex,  the  opposing  but  corresponding  surface  is 
concave.  Each  surface,  therefore,  represents  a  section  of  a  sphere  or 
cylinder,  which  latter  arises  by  rotation  of  a  line  around  an  axis  in  space. 
According  to  the  number  of  axes  around  which  the  movements  take  place 
all  diarthrodial  joints  may  be  divided  into: 

I.  Uniaxial  Joints. — In  this  group  the  convex  articulating  surface  is  a 
segment  of  a  cylinder  or  cone,  to  which  the  opposing  surface  more  or  less 


12  HUMAN  PHYSIOLOGY 

completely  corresponds.  In  such  a  joint  the  single  axis  of  rotation, 
though,  practically  is  not  exactly  at  right  angles  to  the  long  axis  of  the 
bone,  and  hence  the  movements — flexion  and  extension — which  take  place 
are  not  confined  to  one  plane.  Joints  of  this  character — e.g.,  the  elbow, 
knee,  ankle,  the  phalangeal  joints  of  the  fingers  and  toes — are,  therefore, 
termed  ginglymi,  or  hinge-joints.  Owing  to  the  obliquity  of  their  articu- 
lating surfaces,  the  elbow  and  ankle  are  cochleoid  or  screw- ginglymi.  Inas- 
much as  the  axes  of  these  joints  on  the  opposite  sides  of  the  body  are  not 
coincident,  the  right  elbow  and  left  ankle  are  right-handed  screws;  the  left 
elbow  and  right  ankle,  left-handed  screws.  In  the  knee-joint  the  form 
and  arrangement  of  the  articulating  surfaces  are  such  as  to  produce  that 
modification  of  a  simple  hinge  known  as  a  spiral  hinge,  or  helicoid.  As 
the  articulating  surfaces  of  the  condyles  of  the  femur  increase  in  convexity 
from  before  backward,  and  as  the  inner  condyle  is  longer  than  the  outer, 
and  therefore,  represents  a  spiral  surface,  the  line  of  translation  or  the 
movement  of  the  leg  is  also  a  spiral  movement.  During  flexion  of  the  leg 
there  is  a  simultaneous  inward  rotation  around  a  vertical  axis  passing 
through  the  outer  condyle  of  the  femur;  during  extension  a  reverse  move- 
ment takes  place.  Moreover,  the  slightly  concave  articulating  surfaces 
of  the  tibia  do  not  revolve  around  a  single  fixed  transverse  axis,  as  in  the 
elbow-joint,  for  during  flexion  they  slide  backward,  during  extension 
forward,  around  a  shifting  axis,  which  varies  in  position  with  the  point  of 
contact. 

In  some  few  instances  the  long  axis  of  the  articulating  surface  is  parallel 
rather  than  transverse  to  the  long  axis,  and  as  the  movement  then  takes 
place  around  a  more  or  less  conic  surface,  the  joint  is  termed  a  trochoid 
or  pulley — e.g.,  the  odonto-atlantal  and  the  radio-ulnar.  In  the  former 
the  collar  formed  by  the  atlas  and  its  transverse  ligaments  rotates  around 
the  vertical  odontoid  process  of  the  axis.  In  the  latter  the  head  of  the 
radius  revolves  around  its  own  long  axis  upon  the  ulna,  giving  rise  to  the 
movements  of  pronation  and  supination  of  the  hand.  The  axis  around 
which  these  two  movements  take  place  is  continued  through  the  head  of 
the  radius  of  the  styloid  process  of  the  ulna. 

2.  Biaxial  Joints. — In  this  group  the  articulating  surfaces  are  unequally 
curved,  though  intersecting  each  other.  When  the  surfaces  lie  in  the 
same  direction,  the  joint  is  termed  an  ovoid  joint — e.g.,  the  radio-carpal 
and  the  atlanto-occipital.  As  the  axes  of  these  surfaces  are  .vertical  to 
each  other,  the  movements  permitted  by  the  former  joint  are  flexion, 
extension,  adduction,  and  abduction,  combined  with  a  slight  amount  of 
circumduction;  the  latter  joint  permits  of  flexion  and  extension  of  the 


THE  PHYSIOLOGY  OF  THE  SKELETON  1 3 

head,  with  inclination  to  either  side.  When  the  surfaces  do  not  take  the 
same  direction,  the  joint,  from  its  resemblance  to  the  surfaces  of  a  saddle, 
is  termed  a  saddle-joint — e.g.,  the  trapezio-metacarpal.  The  movements 
permitted  by  this  joint  are  also  flexion,  extension,  adduction,  abduction, 
and  circumduction. 

3.  Polyaxial  Joints. — In  this  group  the  convex  articulating  surface  is  a 
segment  of  a  sphere,  which  is  received  by  a  socket  formed  by  the  oppos- 
ing articulating  surface.  In  such  a  joint,  termed  an  enarthrodial  or  ball- 
and-socket  joint — e.g.,  the  shoulder- joint,  hip- joint — the  distal  bone 
revolves  around  an  indefinite  number  of  axes,  all  of  which  intersect  one 
another  at  the  center  of  rotation.  For  simplicity,  however,  the  move- 
ment may  be  described  as  taking  place  around  axes  in  the  three  ordinal 
planes — viz.,  a  transverse,  a  sagittal,  and  a  vertical  axis.  The  move- 
ments around  the  transverse  axis  are  termed  flexion  and  extension; 
around  the  sagittal  axis,  adduction  and  abduction;  around  the  vertical 
axis,  rotation.  When  the  bone  revolves  around  the  surface  of  an  imaginary 
cone,  the  apex  df  which  is  the  center  of  rotation  and  the  base  the  curve 
described  by  the  hand,  the  movement  is  termed  circumduction. 

B.  Amphiarthroses. — In  this  division  are  included  all  these  joints 
which  permit  of  but  slight  movement — e.g.,  the  intervertebral,  the  inter- 
pubic,  and  the  sacro-iliac  joints.  The  surfaces  of  the  opposing  bones  are 
united  and  held  in  position  largely  by  the  intervention  of  a  firm,  elastic 
disc  of  fibro-cartilage.     Each  joint  is  also  strengthened  by  ligaments. 

C  Synarthroses. — In  this  division  are  included  all  those  joints  in  which 
the  opposing  surfaces  of  the  bones  are  immovably  united,  and  hence 
do  not  permit  of  any  movement — e.g.,  the  joints  between  the  bones  of  the 
skull. 

Levers. — In  the  animal  machine,  as  in  physical  machines  generally, 
work  is  accomplished  by  the  intermediation  of  levers.  The  bones  col- 
lectively constitute  a  system  of  levers  the  fulcra  of  which  lie  in  the  joints. 
The  long  bones  more  especially,  are  the  levers  which  are  employed  by  the 
muscles  to  overcome  the  opposing  forces  or  resistances.  The  structure 
and  the  chemic  composition  of  the  bones,  consisting  as  they  do  of  inorganic 
matter  67  per  cent,  and  of  organic  matter  2>S  per  cent,  endow  them  with 
both  rigidity  and  elasticity,  physical  properties  which  admirably  adapt 
them  to  the  character  of  the  work  necessitated  by  the  environment  and 
the  organization  of  the  animal. 

That  a  lever  may  be  effective  as  an  instrument  for  the  accomplishment 
of  work,  it  must  not  only  be  capable  of  moving  around  its  fulcrum,  but 


14  HUMAN  PHYSIOLOGY 

it  must  at  the  same  time  be  acted  on  by  two  opposing  forces,  one  passive, 
the  other  active.  In  the  movement  of  the  bony  levers  of  the  animal 
body,  the  passive  forces  are  largely  those  connected  with  the  environ- 
ment, e.g.,  gravity,  cohesion,  friction,  elasticity,  etc.  The  active  forces 
by  which  these  latter  are  opposed  and  overcome  through  the  intermedia- 
tion of  the  bony  levers  are  found  in  the  muscles  attached  to  them. 

In  all  the  static  and  dynamic  states  of  the  body  the  vertebral  column 
plays  a  most  essential  role.  The  amphiarthrodial  character  of  the 
intervertebral  joint  endows  the  entire  column  with  certain  forms  of 
movement  that  are  necessary  to  the  performance  of  many  body  activities. 

While  the  range  of  movement  between  any  two  vertebrae  is  slight,  the 
sum  total  of  movement  of  the  entire  series  of  vertebrae  is  considerable. 
In  different  regions  of  the  column  the  character,  as  well  as  the  range  of 
movement,  varies  in  accordance  with  the  forms  of  the  vertebrae  and  the 
inclination  of  their  articular  processes.  In  the  cervical  and  lumbar  regions 
extension  and  flexion  are  freely  permitted,  though  the  former  is  greater 
in  the  cervical,  the  latter  in  the  lumbar  region,  especially  between  the 
fourth  and  fifth  vertebrae.  Lateral  flexion  takes  place  in  all  portions  of 
the  column,  but  is  particularly  marked  in  the  cervical  region.  A  rotatory 
movement  of  the  column  as  a  whole  takes  place  through  an  angle  of  about 
twenty-eight  degrees.  This  is  most  evident  in  the  lower  cervical  and 
dorsal  regions. 

The  diarthrodial  character  of  the  joints  of  the  appendicular  portions 
permit  of  extremely  free  movements.  The  character  of  the  movements 
as  well  as  their  extent  depends  largely  on  the  shape  and  adjustment  of 
the  bones  at  their  points  of  union. 

GENERAL  PHYSIOLOGY  OF  MUSCLE  TISSUE 

The  muscle  tissue,  which  closely  invests  the  bones  of  the  body,  and 
which  is  familiar  to  all  as  the  flesh  of  animals,  is  the  immediate  cause 
of  the  active  movements  of  the  body.  This  tissue  is  grouped  in  masses 
of  varying  size  and  shape,  which  are  technically  known  as  muscles.  The 
majority  of  the  muscles  of  the  body  are  connected  with  the  bones  of  the 
skeleton  in  such  a  manner  that,  by  an  alteration  in  their  form,  they  can 
change  not  only  the  position  of  the  bones  with  reference  to  one  another, 
but  can  also  change  the  individual's  relation  to  surrounding  objects. 
They  are,  therefore,  the  active  organs  of  both  motion  and  locomotion, 
in  contradistinction  to  the  bones  and  joints,  which  are  but  passive  agents 
in  the  performance  of  the  corresponding  movements.  In  addition  to  the 
muscle  masses  which  are  attached  to  the  skeleton,  there  are  also  other 


GENERAL  PHYSIOLOGY   OF   MUSCLE   TISSUE  1 5 

collections  of  muscle  tissue  surrounding  cavities  such  as  the  stomach, 
intestine,  blood-vessels,  etc.,  which  impart  to  their  walls  motility,  and 
so  influence  the  passage  of  a  material  through  them. 

Muscles  produce  movement  of  the  structures  to  which  they  are  attached 
by  the  property  with  which  they  are  endowed  of  changing  their  shape, 
shortening  or  contracting  under  the  influence  of  a  stimulus  transmitted 
to  them  from  the  nervous  system.     Muscles  are  therefore  divided  into: 

1.  Skeletal  muscles,  comprising  those  muscles  which  are  attached  to 
the  various  bones  of  the  skeleton. 

2.  Visceral  muscles,  comprising  those  muscles  which  are  found  in  and 
which  compose  a  portion  of  the  walls  of  the  hollow  viscera. 

As  the  skeletal  muscles  are  capable  of  being  excited  to  activity  by  nerve 
impulses  descending  from  the  cerebrum  as  a  result  of  volition  they  are 
frequently  termed  voluntary  muscles.  By  reason  of  their  appearance  as 
seen  under  the  microscope  they  are  termed  also  striped  or  striated  muscles. 
As  the  visceral  muscles  are  not  capable  of  being  excited  to  action  by  voli- 
tion they  are  frequently  termed  involuntary  muscles.  By  reason  of  their 
appearance  as  seen  under  the  microscope  they  are  termed  also 
non-striated  or  smooth  muscles. 

Though  for  the  most  part  the  skeletal  muscles  are  red  in  color,  there  are 
certain  muscles  in  man  and  other  animals  which  are  pale  in  color  and  in 
many  muscles,  pale  fibers  are  extensively  distributed  among  the  red  fibers. 

The  Skeletal  Muscle. — All  skeletal  muscles  consist  of  a  central  fleshy 
portion,  the  body  or  belly,  which  is  provided  at  either  extremity  with 
a  tendon  in  the  form  of  a  cord  or  membrane  by  which  it  is  attached  to  the 
bones.  The  body  is  the  contractile  region,  the  source  of  activity;  the 
tendon  is  a  passive  region,  and  merely  transmits  the  activity  to  the 
bones. 

A  skeletal  muscle  is  a  complex  organ  consisting  of  muscular  fibers, 
connective  tissue,  blood-vessels,  and  lymphatics.  The  general  body  of 
the  muscle  is  surrounded  by  a  dense  layer  of  connective-tissue,  the 
epimysium,  which  blends  with  and  partly  forms  the  tendon;  from  its 
inner  surface  septa  of  connective  tissue  pass  inward  and  group  the  muscle- 
fibers  into  larger  and  smaller  bundles,  termed  fasciculi.  The  fasciculi, 
invested  by  this  special  sheath,  the  perimysium,  are  irregular  in  shape, 
and  vary  considerably  in  size.  The  fibers  of  the  fasciculi  are  separated 
from  one  another  and  supported  by  a  delicate  connective  tissue,  the 
endomysium.  The  connective  tissue  thus  surrounding  and  penetrating 
the  muscle  binds  its  fibers  into  a  distinct  organ,  and  affords  support  to 
blood-vessels,  nerve,  and  lymphatics.     The  muscle  fibers  are  arranged 


1 6  HUMAN  PHYSIOLOGY 

parallel  to  one  another,  and  their  direction  is  that  of  the  long  axis  of  the 
muscle.  In  length  they  vary  from  thirty  to  forty  millimeters,  and  in 
diameter  from  twenty  to  thirty  micromillimeters. 

Histology  of  the  Skeletal  Muscle-Fiber. — A  muscle-fiber  consists  of  a 
transparent  elastic  membrane,  the  sarcolemma,  in  which  is  contained  the 
true  muscle  element.  Examined  microscopically,  the  fiber  presents  a 
series  of  alternate  dim  and  bright  bands,  giving  to  it  a  striated  appearance. 

When  the  bright  band  is  examined  with  high  magnifying  powers,  a  fine, 
dark  line  is  seen  crossing  it  transversely.  It  was  supposed  by  Krause  to  be 
the  optic  expression  of  a  membrane  attached  laterally  to  the  sarcolemma. 

The  muscle-fiber  also  exhibits  a  longitudinal  striation,  indicating  that  it 
is  composed  of  fibrillae,  placed  side  by  side  and  embedded  in  some  inter- 
fibrillar  substance,  to  which  the  name  sarcoplasm  has  been  given.  The 
fibrillae,  which  are  arranged  longitudinally  to  the  long  axis  of  the  fiber,  are 
grouped  by  the  intervening  material  into  bundles  of  varying  size,  the 
muscle  columns.  The  fibrillae  which  extend  throughout  the  length  of  the 
fiber  are  apparently  of  uniform  thickness,  passing  directly  through  the 
transverse  membrane  and  being  supported  by  it. 

In  the  region  of  the  dim  band  the  fibrilla  presents  itself  in  the  form  of  a 
homogeneous  prismatic  rod,  termed  sarcostyle,  separated  from  neighboring 
rods  by  a  slight  amount  of  sacroplasm. 

The  Blood  Supply. — The  blood  supply  to  the  muscle  is  very  great,  and 
the  disposition  of  the  capillary  vessels,  \\ith  reference  to  muscle-fiber,  is 
very  characteristic.  The  arterial  vessels,  after  entering  the  muscle,  are 
supported  by  the  perimysium;  in  this  situation  they  give  off  short,  trans- 
verse branches,  which  immediately  break  up  into  a  capillary  network 
of  rectangular  shape,  within  which  the  muscle-fibers  are  contained.  The 
muscle-fiber  in  intimate  relation  with  the  capillary  is  bathed  with  lymph 
derived  from  it.  Its  contractile  substance,  howev^er,  is  separated  from 
the  lymph  by  its  own  investing  membrane,  through  which  all  interchange 
of  nutritive  and  waste  materials  must  take  place.  Lymphatics  are  pres- 
ent in  muscle,  but  are  confined  to  the  connective  tissue,  in  the  spaces  of 
which  they  have  their  origin. 

The  Nerve  Supply. — The  nerves  which  carry  the  stimuli  to  a  muscle 
enter  near  its  geometric  center.  Many  of  the  fibers  pass  directly  to  the 
muscle-fibers  with  which  they  are  connected;  others  are  distributed  to 
blood-vessels.  Every  muscle-fiber  is  supplied  with  a  special  nerve-fiber, 
except  in  those  instances  where  the  nerve  trunks  entering  a  muscle  do  not 
contain  so  many  fibers  as  the  muscle.  In  such  cases  the  nerve-fibers 
divide,  until  the  number  of  branches  equals  the  number  of  muscle-fibers. 


GENERAL  PHYSIOLOGY  OF   MUSCLE  TISSUE  17 

The  individual  muscle-fiber  is  penetrated  near  its  center  by  the  nerve,  the 
ends  being  practically  free  from  nerve  influence.  The  stimulus  that  comes 
to  the  muscle  fiber  acts  primarily  upon  its  center,  and  then  travels  in  both 
directions  to  the  ends. 

Chemic  Composition  of  Muscle. — The  chemic  composition  of  muscle, 
is  imperfectly  understood,  owing  to  the  fact  that  some  of  its  constitu- 
ents undergo  a  spontaneous  coagulation  after  death,  and  that  the  chemic 
methods  employed  also  tend  to  alter  its  normal  composition.  When 
fresh  muscle  is  freed  from  fat  and  connective  tissue,  frozen,  rubbed  up  in 
a  mortar,  and  expressed  through  linen,  a  slightly  yellow,  syrupy,  alkaline, 
or  neutral  fluid  is  obtained,  known  as  muscle  plasma.  This  fluid  at  nor- 
mal temperature  coagulates  spontaneously,  and  resembles  in  many  re- 
spects the  coagulation  of  blood  plasma.  The  coagulum  subsequently 
contracts  and'  squeezes  out  an  acid  muscle  serum.  The  coagulated  mass 
is  termed  myosin  or  myogen  fibrin.  This  protein  belongs  to  the  class  of 
globulins.  Inasmuch  as  it  is  not  present  in  living  muscle,  and  makes  its 
appearance  only  in  the  as  yet  living  muscle  plasma,  it  is  probable  that  it 
is  derived  from  some  preexisting  substance,  which  is  supposed  to  be 
myosinogen  or  myogen.  Myosin  is  digested  by  pepsin  and  trypsin. 
According  to  Halliburton,  muscle  plasma  contains  the  following  protein 
bodies:  Myosinogen,  paramyosinogen,  albumin,  myoalbumose,  all  of 
which  differ  in  chemic  composition  and  respond  to  various  chemic  and 
physical  reagents. 

Ferment  bodies,  such  as  pepsin  and  diastase;  non-nitrogenized  bodies, 
such  as  glycogen,  lactic  and  sarcolactic  acids,  fatty  bodies,  and  inosite; 
nitrogenized  extractives — e.g.^  urea,  uric  acid,  kreatinin,  as  well  as  in- 
organic salts,  have  been  obtained  from  the  muscle  serum. 

The  Physical  Properties  of  Muscle  Tissue. — The  consistency  of  muscle 
tissue  varies  considerably,  according  to  the  different  states  of  the  muscle. 
In  a  state  of  tension  it  is  hard  and  resistant;  when  free  from  tension,  it  is 
soft  and  fluctuating,  whether  the  muscle  is  contracting  or  resting.  Ten- 
sion alone  produces  hardness.  The  cohesion  of  muscle  tissue  is  less  than 
that  of  connective  tissue,  and  is  broken  more  readily.  Cohesion  resists 
traction  and  pressure,  and  lasts  as  long  as  irritability  remains. 

The  elasticity  of  a  muscle,  though  not  great,  is  almost  perfect.  After 
being  extended  by  a  weight,  it  returns  to  its  natural  form.  The  limit 
of  elasticity,  however,  is  soon  passed.  A  weight  of  50  or  loo  grams  will 
overcome  the  elasticity  so  that  it  will  not  return  to  its  natural  length.  In 
inorganic  bodies  the  extension  is  directly  proportional  to  the  extending 
weight,  and  the  line  of  extension  is  straight.  With  muscles,  the  extension 
2 


1 8  HUMAN  PHYSIOLOGY 

is  not  proportional  to  the  weight.  While  at  first  it  is  marked,  the  elonga- 
tion diminishes  as  the  weight  increases  by  equal  increments,  so  that  the 
line  of  extension  becomes  a  curve.  In  other  words,  the  elasticity  of  a  pas- 
si  ve  muscle  augments  with  increased  extension.  On  the  contrary  the 
elasticity  of  an  active  is  less  than  that  of  a  passive  muscle,  for  it  is  elon- 
gated more  by  the  same  weight,  as  shown  by  experiment. 

Tonicity  is  a  property  of  all  muscles  in  the  body,  in  consequence  of  being 
normally  stretched  to  a  slight  extent  beyond  their  natural  length.  This 
may  be  due  to  the  action  of  antagonistic  muscles,  or  to  the  elasticity  of  the 
parts  of  the  skeleton  to  which  they  are  attached.  This  is  shown  by  the 
shortening  of  the  muscle  which  takes  place  when  it  is  divided.  Muscular 
tonus  plays  an  important  r61e  in  muscular  contraction.  Being  always  on 
the  stretch,  the  muscle  loses  no  time  in  acquiring  that  degree  of  tension 
necessary  to  its  immediate  action  on  the  bones.  Again,  the  working 
power  of  a  muscle  is  increased  by  the  presence  of  some  resistance  to  the 
act  of  contraction.  According  to  Marey,  the  amount  of  work  is  con- 
siderably increased  when  the  muscular  energy  is  transmitted  by  an 
elastic  body  to  the  mass  to  be  moved,  while  at  the  same  time,  the  shock 
of  the  contraction  is  lessened.  The  position  of  a  passive  limb  is  the  result- 
ant also  of  the  elastic  tension  of  antagonistic  groups  of  muscles. 

Muscle  excitability  and  contractility  are  terms  employed  to  denote  that 
property  of  muscle  tissue  in  virtue  of  which  it  contracts  or  shortens  in 
response  to  various  excitants  or  stimuli.  Though  usually  associated  with 
the  activity  of  the  nervous  system,  it  is  nevertheless  an  independent  en- 
dowment, and  persists  after  all  nervous  connections  are  destroyed.  If  the 
nerve  terminals  be  destroyed,  as  they  can  be  by  the  introduction  of  curara 
into  the  system,  the  muscles  become  completely  relaxed  and  quiescent. 
The  strongest  stimuli  applied  to  the  nerves  fail  to  produce  a  contraction. 
Various  external  stimuli  applied  directly  to  the  muscle  substance  produce 
at  once  the  characteristic  contraction.  The  excitability  of  muscle  is  there- 
fore an  inherent  property,  dependent  on  its  nutrition,  and  persisting  as 
long  as  it  is  supplied  with  proper  nutritive  materials  and  surrounded  by 
those  external  conditions  which  maintain  its  chemic  or  physical  integrity. 

Muscle  Contractions. — All  muscle  contractions  occurring  in  the  body 
under  normal  physiologic  conditions  are  either  voluntary,  caused  by  a 
volitional  effort  and  the  transmission  of  a  nerve  impulse  from  the  brain 
through  the  spinal  cord  and  nerves  to  the  muscles,  or  reflex,  caused  by  a 
peripheral  stimulation  and  the  transmission  of  a  nerve  impulse  to  the 
spinal  cord,  to  be  reflected  outward  through  the  same  nerves  to  the  mus- 
cles.    In  either  case  the  resulting  contraction  is  essentially  the  same.     The 


GENERAL  PHYSIOLOGY   OF   MUSCLE   TISSUE  1 9 

normal  or  physiologic  stimulus  which  provokes  the  muscular  contraction 
is  a  nerve  impulse  the  nature  of  which  is  unknown,  but  is  perhaps  allied 
to  a  molecular  disturbance.  After  removal  from  the  body,  muscles 
remain  in  a  state  of  rest,  inasmuch  as  they  possess  no  spontaneity  ot 
action.  Though  consisting  of  a  highly  irritable  tissue,  they  cannot  pass 
from  the  passive  to  the  active  state  except  upon  the  application  of  some 
form  of  stimulation. 

The  stimuli  which  are  capable  of  calling  forth  a  contraction  may  be 
divided  into : 

I.  Mechanical.     2.  Chemic.     3.  Physical.     4.  Electric. 

Every  mechanical  stimulus  of  a  muscle — e.g.,  pick,  cut,  or  tap — pro- 
vided it  has  sufficient  intensity,  and  is  repeated  with  sufficient  rapidity, 
will  cause  not  only  a  single  contraction,  but  a  series  of  contractions. 

All  chemic  agents  which  impair  the  chemic  composition  of  the  muscle 
with  sufficient  rapidity — e.g.,  hydrochloric  acid,  acetic  and  oxalic  acids, 
distilled  water  injected  into  the  vessels,  etc. — act  as  stimuli,  and  produce 
single  and  multiple  contractions.  Physical  agents,  as  heat  and  elec- 
tricity, also  act  as  stimuli.  A  muscle  heated  rapidly  to  3o°C.  contracts 
vigorously,  and  reaches  its  maximum  at  45° C.  Of  all  forms  of  stimuli, 
the  electric  is  the  most  generally  used.  Two  forms  are  used — the  induced 
current  and  the  make-and-break  of  a  constant  current. 

Changes  in  a  Muscle  During  Contraction. — When  a  muscle  is  stimu- 
lated, either  indirectly  through  the  nerve  or  directly  by  any  external 
agent,  it  undergoes  a  series  of  changes,  which  relate  to  its  form,  volume, 
optic,  physical  chemic,  and  electric  properties.  These  changes,  in  their 
totality,  constitute  the  muscular  contraction. 

1.  Form.— The  most  obvious  change  is  that  of  form.  The  fibers  become 
shorter  in  their  longitudinal  and  wider  in  their  transverse  diameters,  and 
the  muscle  as  a  whole  becomes  shorter  and  thicker.  The  degree  of 
shortening  may  amount  to  thirty  per  cent,  of  the  original  length. 

2.  Volume. — The  increase  in  transverse  diameter  does  not  fully  com- 
pensate for  the  diminution  in  length,  for  there  is  at  the  moment  of  con- 
traction a  slight  shrinkage  in  volume,  which  has  been  attributed  to  a 
compression  of  air  in  its  interstices. 

3.  Optic  Changes. — If  a  muscle-fiber  be  examined  microscopically  dur- 
ing its  contraction,  it  will  be  observed  that  when  the  contraction  wave 
begins,  both  bright  and  dim  bands  diminish  in  height  and  become  broader, 
though  this  change  is  more  noticeable  in  the  region  of  the  bright  band. 
This  Englemann  attributes  to  a  passage  of  fluid  material  from  the  bright 


20  HUMAN  PHYSIOLOGY 

into  the  dim  band.  At  the  time  of  relaxation  there  is  a  return  of  this 
material,  and  the  fiber  assumes  its  original  shape  and  volume.  As  the 
contraction  wave  reaches  its  maximum,  the  optic  properties  of  both  the 
isotropic  and  anisotropic  bands  change.  The  former,  which  was  origi- 
nally clear,  now  becomes  darker  and  less  transparent,  until  at  the  crest  of 
the  wave  it  assumes  the  appearance  of  a  distinct  dark  band.  The  latter , 
the  anisotropic,  which  was  originally  dim,  now  becomes,  in  comparison, 
clear  and  light.  This  change  in  optic  appearance  is  due  to  an  increase  in 
refrangibility  of  the  isotropic  and  a  decrease  in  the  anisotropic  bands 
coincident  with  the  passage  of  fluid  from  the  former  into  the  latter.  TJiere 
is  at  the  height  of  the  contraction  a  complete  reversal  in  the  positions^of 
the  striations.  At  a  certain  stage  between  the  beginning  and  the  crest 
of  the  wave  there  is  an  intermediate  point,  at  which  the  striae  almost 
entirely  disappear,  giving  to  the  fiber  an  appearance  of  homogeneity. 
There  is,  however,  no  change  in  refractive  power,  as  shown  by  the  polar- 
izing apparatus.  After  the  contraction  wave  has  reached  the  stage  of 
greatest  intensity,  there  is  a  reversal  of  the  foregoing  phenomena,  and 
the  fiber  returns  to  its  original  condition,  which  is  one  of  relaxation. 

4.  Physical  Changes. — The  extensibility  of  muscle  is  increased  during  the 
contraction,  the  same  weight  elongating  the  fibers  to  a  greater  extent 
than  during  rest.  The  elasticity,  or  its  power  of  returning  to  its  original 
form,  is  correspondingly  diminished. 

5.  Chemic  Changes. — The  chemic  changes  which  take  place  in  a  muscle 
during  contraction  or  activity  are  very  complex. 

As  shown  by  an  analysis  of  the  blood  flowing  to  and  from  the  resting 
muscle,  it  has,  while  passing  through  the  capillaries,  lost  oxygen  and 
gained  carbon  dioxid.  The  amount  of  oxygen  absorbed  by  the  muscle 
(nine  per  cent.)  is  greater  than  the  amount  of  CO2  given  off  (6.7  per  cent.). 
There  is  no  parallelism  between  these  two  processes,  as  CO2  will  be  given 
off  in  the  absence  of  oxygen,  or  in  an  atmosphere  of  nitrogen. 

In  the  active  or  contracting  muscle  both  the  absorption  of  oxygen  and 
the  production  of  CO2  are  largely  increased,  but  the  ratio  existing  between 
them  differs  considerably  from  that  of  the  resting  muscle,  for  the  quantity 
of  oxygen  absorbed  amounts  to  11.26  per  cent,  the  quantity  of  CO2  to 
10.8  per  cent.  (Ludwig).  Moreover,  in  a  tetanized  muscle  the  quantity 
of  CO2  given  off  may  be  largely  in  excess  of  the  oxygen  absorbed.  From 
these  facts  it  is  evident  that  the  energy  of  the  contraction  does  not  depend 
upon  the  direct  oxidation  of  certain  substances,  but  upon  the  decomposi- 
tion of  some  unstable  compound  of  high  potential  energy,  rich  in  carbon 
and  oxygen.     When  the  muscle  is  active,  its  tissue  changes  from  a  neutral 


GENERAL  PHYSIOLOGY   OF   MUSCLE   TISSUE  21 

to  an  acid  reaction,  from  the  development  of  sarcolactic  and  possibly 
phosphoric  acids.  The  amount  of  glycogen  present  in  muscle  (0.43  per 
cent.)  diminishes,  but  muscles  wanting  in  glycogen,  nevertheless,  retain 
their  power  of  contraction.  Water  is  absorbed.  The  amount  of  urea  is 
not  materially  increased  by  muscular  activity,  unless  it  is  excessive  and 
prolonged,  and  then  only  in  the  absence  of  a  sufficient  quantity  of  non- 
nitrogenized  material.  Coincident  with  muscle  contraction,  the  blood- 
vessels become  widely  dilated,  leading  to  a  large  increase  in  the 
blood-supply  and  a  rapid  removal  of  products  of  decomposition. 

Thermic  Changes. — Coincident  with  the  foregoing  chemic  changes  and 
the  transformation  of  energy,  there  is  a  liberation  of  heat  and  a  rise  in 
the  temperature  of  the  muscle.  A  single  contraction  of  the  gastrocne- 
mius muscle  of  the  frog,  will  raise  its  temperature  0.00  i°C. 

Electric  phenomena  also  manifest  themselves  which  are  similar  to 
the  electric  phenomena  presented  by  nerves  and  will  be  alluded  to  in  a 
subsequent  section. 

Transmission  of  the  Contraction  Wave. — Normally,  when  a  muscle  is 
stimulated  by  the  nerve  impulse,  the  shortening  and  thickening  of  the 
fibers  begin  at  the  end  organ  and  travel  in  opposite  directions  to  the  ends 
of  the  muscle.  This  change  propagates  itself  in  a  wave-like  manner, 
and  has  been  termed  the  contraction  wave.  If  a  stimulus  be  applied 
directly  to  the  end  of  a  long  muscle,  the  contraction  wave  passes  along 
its  entire  length  to  the  opposite  extremity,  in  virtue  of  the  conductivity 
of  muscular  tissue.  The  rapidity  of  the  propagation  varies  in  different 
animals — in  the  frog,  from  three  to  four  meters  a  second,  in  man,  from 
ten  to  thirteen  meters.  The  length  of  the  wave  varies  from  200  to  400 
millimeters. 

Graphic  Record  of  a  Muscle  Contraction. — The  changes  in  the  form 
of  a  muscle  during  contraction  and  relaxation  have  been  carefully  studied 
by  recording  the  muscle  movement  by  means  of  an  attached  lever,  the 
end  of  which  is  allowed  to  rest  upon  a  moving  surface.  The  time  rela- 
tions of  all  phases  of  the  muscular  movement  are  obtained  by  placing 
beneath  the  lever  a  pen  attached  to  an  electro-magnet  thrown  into  action 
by  a  tuning-fork  vibrating  in  hundredths  of  a  second.  A  marking  lever 
records  simultaneously  the  moment  of  stimulation. 

Single  Contraction. — When  a  single  electric  induction  shock  is  applied 
to  a  nerve  close  to  the  muscle,  the  latter  undergoes  a  quick  pulsation, 
speedily  returning  to  its  former  condition.  As  shown  by  the  muscle 
curve  (see  Fig.  i)  there  is  between  the  moment  of  stimulation  and  the 


22  HUMAN  PHYSIOLOGY 

beginning  of  the  contraction  a  short  but  measurable  period,  known  as 
the  latent  period,  during  which  certain  chemic  changes  are  taking  place 
preparatory  to  the  exhibition  of  the  muscle  movement.  Even  when  the 
electric  stimulus  is  applied  directly  to  the  muscle,  a  latent  period,  though 
shorter,  is  observable.  The  duration  of  this  period  in  the  skeletal  muscles 
of  the  frog  has  been  estimated  at  o.oi  of  a  second;  but  it  has  been  shown 
by  the  employment  of  more  accurate  methods  and  the  elimination  of 
various  external  influences  to  be  much  less — not  more  than  0.0033  to 
0.0025  of  ^  second. 

The  contraction  follows  the  latent  period.  This  begins  slowly,  rapidly 
reaches  its  maximum,  and  ceases.  This  has  been  termed  the  stage  of 
rising  or  increasing  energy.  The  time  occupied  in  the  stage  of  shortening 
is  about  0.04  of  a  second,  though  this  will  depend  on  the  strength  of  the 
stimulus,  the  load  with  which  the  muscle  is  weighted,  and  the  condition 
of  the  muscle  irritability. 


Fig.  I. — Muscle  Curve  Produced  by  a  Single  Induction  Shock  Applied  to 

A  Muscle. — (Landois.) 
a-f.  Abscissa,     a-c.  Ordinate,     a-b.  Period  of  latent  stimulation,     b-d.  Period  of 
increasing  energy,     d-e.  Period  of  decreasing  energy,     e-f.  Elastic  after-vibrations. 

The  relaxation  immediately  follows  the  contraction.  This  takes  place 
at  first  slowly,  after  which  the  muscle  rapidly  returns  to  its  original 
length.  This  is  the  period  of  falling  or  decreasing  energy,  and  occupies 
about  0.05  of  a  second.  The  whole  duration  of  a  muscle  contraction 
occupies,  therefore,  about  o.i  of  a  second. 

Residual  or  after- vibrations  are  frequently  seen  which  are  due  to  changes 
in  the  elasticity  of  the  muscle.  The  amplitude  of  the  contraction  depends 
upon  the  condition  of  the  muscle,  the  load,  the  strength  of  stimulus,  etc. 

Action  of  Successive  Stimuli. — If  a  series  of  successive  stimuli  be  applied 
to  a  muscle,  the  effect  will  be  different  according  to  the  rapidity  with 
which  they  follow  one  another.  If  the  second  stimulus  be  applied  at  the 
termination  of  the  contraction  due  to  the  first  stimulus,  a  second  con- 
traction follows,  similar  in  all  respects  to  the  first.  A  third  stimulus 
produces  a  third  contraction,  and  so  on  until  the  muscle  becomes  ex- 


GENERAL   PHYSIOLOGY   OF    MUSCLE    TISSUE  23 

hausted.  If  the  second  stimulus  be  applied  during  either  of  the  two 
periods  of  the  first  contraction,  the  effects  of  the  two  stimuli  will  be  added 
together  and  the  second  contraction  will  add  itself  to  the  first.  The 
maximum  contraction  is  obtained  when  the  second  stimulus  is  applied 
J^o  of  a  second  after  the  first. 

Tetanus. — Tetanus  may  be  defined  as  a  more  or  less  continuous  con- 
traction of  a  muscle  which  arises  when  the  time  intervals  between  the 
stimuli  are  shorter  than  the  time  of  the  contraction  process.  Tetanus 
will  be  incomplete  or  complete  according  to  the  number  of  stimuli  that 
reach  the  muscle  in  a  second  of  time.  When  a  muscle  is  stimulated 
directly  or,  better,  indirectly  through  its  related  nerve  by  a  series  of  in- 
duced currents  at  the  rate  of  four  or  six  per  second,  it  undergoes  a 
corresponding  number  of  contractions  of  about  equal  extent.  If  the  rate 
of  stimulation  is  increased  up  to  the  point  when  the  interval  between  each 
stimulus  is  less  than  the  duration  of  the  entire  contraction  process,  the 
muscle  does  not  have  time  to  relax  completely  before  the  arrival  of  the 
succeeding  stimulus,  and  hence  remains  in  a  more  or  less  contracted  state, 
during  which  it  exhibits  a  series  of  alternate  partial  contractions  and 
relaxations.  To  this  condition  of  muscle  activity  the  term  incomplete 
tetanus  or  clonus  is  applied. 

If  the  stimulation  be  still  further  increased  in  frequency,  the  individual 
contractions  become  fused  together  and  the  curve  described  by  the  lever 
becomes  a  continuous  line.  Notwithstanding  the  fact  that  the  individual 
contractions  are  no  longer  visible,  it  can  be  shown  by  other  methods  that 
the  muscle  is  undergoing  a  series  of  slight  alternate  contractions  and  relaxa- 
tions or  vibrations  at  least.  After  a  varying  length  of  time  the  muscle 
becomes  fatigued,  relaxes,  and  returns  to  its  natural  condition  even  though 
the  stimulation  be  continued.  The  number  of  stimuli  per  second  neces- 
sary to  develop  complete  tetanus  will  depend  under  normal  circumstances 
on  the  period  of  duration  of  the  individual  contractions.  The  longer  this 
period,  the  less  the  number  of  stimuli  required,  and  the  reverse.  Hence 
the  number  of  stimuli  will  vary  for  different  classes  of  animals  and  for 
different  muscles  in  the  same  animal,  e.g.,  2  or  3  for  the  muscles  of  the 
tortoise,  10  for  the  muscles  of  the  rabbit,  15  to  20  for  the  frog,  70  to  80  for 
birds,  330  to  340  for  insects. 

Physiologic  Tetanus. — A  physiologic  tetanus  of  longer  or  shorter 
duration  may  be  established  by  an  act  of  volition  or  by  the  action  of  some 
external  stimulus.  In  the  first  instance  the  tetanus  is  termed  volitional 
and  in  the  second  instance,  reflex. 


24  HUMAN  PHYSIOLOGY 

1.  Volitional  tetanus.  As  the  volitional  contraction  is  similar  to  that 
observed  when  a  muscle  or  its  related  nerve  is  stimulated  by  rapidly 
repeated  induced  currents,  it  is  assumed  that  the  nerve-cells  in  the  spinal 
cord  are  discharging  in  a  rhythmic  manner  a  certain  number  of  nerve 
impulses  per  second  in  consequence  of  the  arrival  of  nerve  impulses  coming 
from  the  cerebral  cortex,  the  result  of  volitional  acts.  In  other  words 
the  Volitional  tetanus  is  the  result  of  a  discontinuous  stimulation.  The 
number  of  stimuli  transmitted  to  a  muscle  during  a  volitional  tetanus  has 
been  estimated  by  the  employment  of  the  graphic  method  at  from  8  to  13 
per  second,  10  being  about  the  average.  When  a  volitional  contraction 
is  recorded  the  myogram  not  infrequently  exhibits  a  series  of  small  wave- 
like elevations  which  indicate  that  the  muscle  is  not  in  a  state  of  complete 
tetanus  but  is  undergoing  slight  alternate  contractions  and  relaxations. 
Unless  the  contraction  process  in  human  muscle  differs  from  that  of  frogs 
it  is  difficult  to  see  how  10  or  even  20  stimuli  per  second  can  give  rise  to 
even  an  incomplete  tetanus  when  the  single  contraction  is  J^o  of  a  second 
in  duration. 

2.  Reflex. — A  tetanus  of  muscle,  physiologic  in  character,  arises  during 
the  performance  of  many  muscle  movements  in  consequence  of  peripherally 
acting  causes  and  may  therefore  be  termed  a  reflex  tetanus.  The  dura- 
tion of  a  tetanus  thus  induced,  like  the  duration  of  a  volitional  tetanus, 
will  vary  with  the  duration  of  the  exciting  cause.  Reflex  tetani  are  pre- 
sented by  the  muscles  of  the  lower  jaw  during  mastication,  by  the  inter- 
costal muscles  during  breathing,  by  the  muscles  of  the  limbs  during 
walking,  etc.  In  these  and  other  instances  there  are  reasons  for  believing 
that  for  a  variable  period  of  time  the  muscles  are  in  a  state  of  continuous 
contraction  from  the  discharge  of  nerve  impulses  from  the  nerve  cells  in 
the  spinal  cord  as  the  result  of  the  arrival  of  nerve  impulses  coming  from 
a  peripheral  surface. 

A  non-physiologic  tetanus  may  be  excited  or  developed  by  the  action  of 
pharmacologic  agents,  e.g.y  strychnin,  and  of  pathologic  agents,  e.g.,  toxins 
developed  by  bacteria,  acting  on  the  spinal  cord  mechanisms. 

Muscle  Fatigue. — Prolonged  or  excessive  muscular  activity  is  followed 
by  a  diminution  in  the  power  of  performing  work  and  by  an  increase  in  the 
duration  of  the  muscular  contractions.  Fatigue  is  accompanied  by  a  feel- 
ing of  stiffness,  soreness,  and  lassitude,  referable  to  the  muscles  themselves. 
In  the  early  stages  of  muscular  fatigue  the  contractions  increased  in  height 
and  duration,  to  be  followed  by  a  progressive  decrease  in  height,  but  an 
increase  in  duration,  until  the  muscle  becomes  exhausted.  The  cause  of 
the  fatigue  is  the  production  and  accumulation  of  decomposition  products, 


GENERAL  PHYSIOLOGY   OF   MUSCLE   TISSUE  25 

such  as  phosphoric  acid  and  phosphate  of  potassium,  CO2,  etc.  A  fatigued 
muscle  is  rapidly  restored  by  the  injection  of  arterial  blood. 

The  Source  of  the  Energy  and  the  Nature  of  the  Muscle  Contraction. — 

The  passage  of  a  nerve  impulse  into  a  muscle  together  with  its  subsequent 
action,  calls  forth  a  pulsation,  which  is  attended  with  the  production  of 
lactic  acid,  carbon-dioxid  and  water  and  the  liberation  of  heat.  These 
phenomena  would  indicate  that  some  compound  had  undergone  an  oxida- 
tion in  whole  or  in  part.  The  exact  chemic  nature  of  the  compound  has 
however  not  been  determined.  Whatever  the  nature  of  the  compound  the 
problem  requiring  solution  is  how  do  the  chemic  changes  and  the  con- 
comitant liberation  of  energy  cause  the  muscle  to  contract.  The  latest 
explanation  is  that  of  Hill.  This  investigator  assumes  that  there  is  pri- 
marily in  the  muscle  a  complex  compound  the  exact  nature  of  which  has 
not  been  determined  but  which  contains  potential  energy.  The  arrival 
of  the  nerve  impulse  leads  to  a  disruption  of  this  compound  with  the  libera- 
tion of  lactic  acid.  The  acid  at  once  acidifies  the  sarcous  elements  and  in 
so  doing  endows  them  with  the  power  of  imbibing  water  from  the  sarco- 
plasm,  whereupon  they  swell  and  tend  to  approximate  a  spherical  shape 
and  thus  shorten  the  muscle.  Following  the  contraction  or  the  shortening, 
there  occurs  an  oxidation  of  sugar  with  the  production  of  carbon  dioxid 
and  water  and  the  liberation  of  heat.  A  portion  of  the  heat  is  then  utilized 
in  the  re-formation  or  reconstruction  of  the  compound.  The  lactic  acid 
is  again  incorporated  in  whole  or  in  part  and  the  heat  absorbed  is  trans- 
formed into  potential  or  chemical  energy.  With  the  withdrawal  of  the 
acid,  the  sarcous  elements  lose  their  imbibition  power  and  the  fluid  returns 
to  the  surrounding  sarcoplasm. 

The  Production  of  Heat  and  Its  Relation  to  Mechanical  Work.— -The 

transformation  of  energy  which  takes  place  during  a  muscle  contraction, 
and  which  is  dependent  upon  chemic  changes  occurring  at  that  time,  mani- 
fests itself  as  heat  and  mechanical  work.  While  heat  is  being  evolved  con- 
tinuously during  the  passive  condition  of  muscles,  the  amount  of  heat  is 
largely  increased  during  general  muscle  contraction.  A  skeletal  muscle  of 
a  frog — e.g.,  the  gastrocnemius — when  removed  from  the  body,  shows, 
after  tetanization,  an  increase  in  its  temperature  of  from  0.14°  to  o.i8°C., 
and  after  a  single  contraction  of  from  0.001°  to  0.005 °C.  While  every 
muscular  contraction  is  attended  by  an  increase  in  heat  production,  the 
amount  so  produced  will  vary  in  accordance  with  certain  conditions — e.g., 
tension,  work  done,  fatigue,  circulation  of  blood,  etc. 

Tension. — The  greater  the  tension  of  a  muscle,  the  greater,  other  condi- 
tions being  equal,  is  the  amount  of  heat  evolved.    When  the  ends  of  a 


26  HUMAN   PHYSIOLOGY 

muscle  are  fastened  so  that  no  shortening  is  possible  during  stimulation, 
the  maximum  of  heat  production  is  reached.  In  the  tetanic  state  the 
great  increase  of  temperature  is  due  to  the  tension  of  antagonistic  and 
strongly  contracted  muscles.  The  evolution  of  heat,  therefore,  bears  a 
relation  to  the  resistance  against  which  the  muscle  is  acting. 
Mechanical  Work. — If  a  muscle  contracts,  loaded  by  a  weight  just  suffi- 
.  cient  to  elongate  it  to  its  original  length,  heat  is  evolved,  but  no  mechanical 
work  is  done,  all  the  energy  liberated  manifesting  itself  as  heat.  When  the 
weight  which  has  been  lifted  is  removed  from  the  muscle  at  the  height  of 
contraction,  external  work  is  done.  In  this  case  the  amount  of  heat 
liberated  is  less,  owing  to  the  work  done,  for  some  of  the  heat  generated  is 
transformed  into  mechanical  motion.  According  to  the  law  of  the  con- 
servation of  energy,  the  amount  of  heat  disappearing  should  correspond 
in  heat  units  to  the  number  of  foot-pounds  produced  by  muscular  con- 
traction. 

Work  Done. — Muscles  are  machines  capable  of  doing  a  certain  amount 
of  work,  by  which  is  meant  the  raising  of  a  weight  against  gravity  or  the 
overcoming  of  some  resistance.  The  work  done  is  calculated  by  multiply- 
ing the  weight  by  the  distance  through  which  it  is  raised.  Thus,  if  a 
muscle  shortens  four  millimeters  and  raises  250  grams,  it  does  work  equal 
to  1,000  milligramme ters,  or  one  gram-meter.  If  a  muscle  contracts 
without  being  weighted,  no  work  is  done.  Equally,  when  the  muscle  is 
over- weigh  ted  so  that  it  is  unable  to  contract,  no  work  is  done.  The 
amount  of  work  a  muscle  can  do  will  depend  upon  the  area  of  its  trans- 
verse section,  the  length  of  its  fibers,  and  the  amount  of  the  weight.  The 
amount  of  work  a  laborer  of  70  kilograms  weight  performs  in  eight  hoiirs 
averages  105,605  kilogram-meters,  or  340.2  foot- tons. 

Muscle  Sound. — Providing  a  muscle  be  kept  in  a  state  of  tension  during 
its  contraction,  the  intermittent  variations  of  its  tension  cause  the  muscle 
to  emit  an  audible  sound.  If  the  muscle  be  tetanized  by  induction  shocks, 
the  pitch  of  the  sound  corresponds  with  the  number  of  stimuli  a  second. 
A  voluntary  contraction  is  attended  by  a  tone  having  a  vibration  fre- 
quency of  about  thirty-six  a  second,  which  is,  however,  the  first  overtone 
of  the  true  muscle  tone,  which  is  caused  by  a  contraction  frequency  of 
about  eighteen  a  second.  This  low  tone  is  inaudible,  from  the  small 
number  of  vibrations  a  second. 

Rigor  Mortis. — A  short  time  after  death  the  muscles  pass  into  a  condi- 
tion of  extreme  rigidity  or  contraction,  which  lasts  from  one  to  five  days. 
In  this  state  they  offer  great  resistance  to  extension,  their  tonicity  dis- 
appears, their  cohesion  diminishes,  their  irritability  ceases.     The  time  of 


GENERAL  PHYSIOLOGY   OF   MUSCLE  TISSUE  27 

the  appearance  of  this  post-mortem  or  cadaveric  rigidity  varies  from  a 
quarter  of  an  hour  to  seven  hours.  Its  onset  and  duration  are  influenced 
by  the  condition  of  the  muscular  irritability  at  the  time  of  death.  When 
the  irritability  is  impaired  from  any  cause,  such  as  disease  or  defective 
blood-supply,  the  rigidity  appears  promptly,  but  is  of  short  duration. 
After  death  from  acute  diseases,  it  is  apt  to  be  delayed,  but  to  continue 
for  a  longer  period. 

The  rigidity  appears  first  in  the  muscles  of  the  lower  jaw  and  neck;  next 
in  the  muscles  of  the  abdomen  and  upper  extremities;  finally  in  the  trunk 
and  lower  extremities.    It  disappears  in  practically  the  same  order. 

Chemic  changes  of  a  marked  character  accompany  this  rigidity.  The 
muscle  becomes  acid  in  reaction  from  the  development  of  sarcolactic  acid; 
it  gives  off  a  large  quantity  of  carbonic  acid,  and  is  shortened  and  dimin- 
ished in  volume. 

The  immediate  cause  of  the  rigidity  appears  to  be  a  coagulation  of  the 
myosinogen  within  the  sarcolemma,  with  the  subsequent  formation  of 
myosin  and  muscle  serum.  In  the  early  stages  of  coagulation  restitution 
is  possible  by  the  circulation  of  arterial  blood  through  the  vessels.  The 
final  disappearance  of  this  contraction  is  due  to  the  action  of  acids  dis- 
solving the  myosin,  and  possibly  to  putrefactive  changes. 

The  Visceral  Muscle. — The  visceral  muscle,  as  the  name  implies,  is 
found  in  the  walls  of  hollow  viscera,  where  it  is  arranged  in  the  form  of  a 
membrane  or  sheet.  It  is  present  in  the  walls  of  the  alimentary  canal, 
blood-vessels,  respiratory  tract,  ureter,  bladder,  vas  deferens,  uterus, 
fallopian  tubes,  iris,  etc.  In  some  situations  it  is  especially  thick  and  well 
developed — e.g.,  uterus  and  pyloric  end  of  the  stomach;  in  others  it  is  thin 
and  slightly  developed. 

The  Histology  of  the  Visceral  Muscle-fiber. — When  examined  with 
the  microscope,  the  muscle  sheet  is  seen  to  be  composed  of  fibers,  narrow, 
elongated,  and  fusiform  in  shape.  As  a  rule,  they  are  extremely  small, 
measuring  only  from  40  to  250  micromillimeters  in  length  and  from  4  to  8 
micromillimeters  in  breadth.  The  center  of  each  fiber  presents  a  narrow, 
elongated  nucleus.  The  muscle-protoplasm  which  makes  up  the  body  of 
the  fiber  appears  to  be  enclosed  by  a  delicate  elastic  membrane  resembling 
in  some  respects  the  sarcolemma  of  the  skeletal  muscle.  In  some  animals 
the  visceral  fiber  presents  a  longitudinal  stria tion  suggesting  the  existence 
of  fibrillae  surrounded  by  sarcoplasm.  The  fibers  are  united  longitudi- 
nally and  transversely  by  a  cement  material.  The  muscle  is  increased  in 
thickness  by  the  superposition  of  successive  layers.  At  varying  intervals 
the  fibers  are  grouped  into  bundles  or  fasciculi  by  septa  of  connective 


28  *      HUMAN  PHYSIOLOGY 

tissue.     Blood-vessels  ramify  in  the  connective  tissue  and  furnish  the 
necessary  nutritive  material. 

The  visceral  muscle  receives  stimuli  from  the  spinal  cord,  not  directly, 
however,  as  in  the  case  of  the  skeletal  muscle,  but  indirectly  through  the 
intermediation  of  ganglion  cells,  which  may  be  located  at  some  distance 
from  the  muscle  or  near  the  walls  of  the  viscera.  Non-medullated  fibers 
from  the  ganglion  pass  directly  into  the  muscle,  where  they  frequently 
unite  to  form  a  general  plexus.  From  this  plexus  fine  branches  take  their 
origin  and  ultimately  become  physiologically  associated  with  the  muscle- 
fiber. 

Physiologic  Properties. — The  physiologic  properties  of  visceral  muscles 
are  tonicity,  elasticity,  conductivity  and  irritability,  properties  which 
closely  resemble  the  corresponding  properties  of  the  skeletal  muscles. 

A  contraction  of  the  visceral  muscle  can  be  called  forth  by  the  passage 
of  a  single  induced  current  and  which  can  be  graphically  recorded.  The 
duration  of  the  contraction  is.  however,  very  much  longer  than  the  dura- 
tion of  the  skeletal  muscle  contraction;  thus  the  period  of  shortening  may 
last  for  five  seconds  and  the  period  of  relaxation  for  as  much  as  thirty- 
five  seconds.  The  muscle  can  also  be  tetanized.  Moreover  it  will  respond 
to  variations  in  temperature,  strength  of  stimulus,  to  the  load  in  a  manner 
similar  to  if  not  identical  with  the  skeletal  muscle. 

The  Fimction  of  the  Visceral  Muscle. — In  a  general  way  it  may  be 
said  that  the  visceral  muscle  determines  and  regulates  the  passage  through 
the  viscus  or  organ  of  the  material  contained  within  it.  'Hie  food  in  the 
stomach  and  intestines  is  subjected  to  a  churning  process  by  the  muscles, 
in  consequence  of  which  the  digestive  fluids  are  more  thoroughly  incor- 
porated and  their  characteristic  action  increased.  At  the  same  time  the 
food  is  carried  through  the  canal,  the  absorption  of  the  nutritive  material 
promoted,  and  the  indigestible  residue  removed  from  the  body.  The 
blood  is  delivered  in  larger  or  smaller  volumes  according  to  the  needs  of 
the  tissues  through  a  relaxation  or  contraction  of  the  muscle-fibers  of  the 
blood-vessels.  The  urine  is  forced  through  the  ureter  and  from  the 
bladder  by  the  contraction  of  their  respective  muscles.  The  mode  of 
action  of  the  individual  muscles  will  be  described  in  subsequent  chapters. 

SPECIAL  PHYSIOLOGY  OF  MUSCLES 

The  individual  muscles  of  the  axial  and  appendicular  portions  of  the 
body  are  named  with  reference  to  their  shape,  action,  structure,  etc. — 
e.g.,  deltoid,   flexor,  penniform,  etc.     In  different  localities  a  group  of 


SPECIAL   PHYSIOLOGY   OF   MUSCLES  -29 

muscles  having  a  common  function  is  named  in  accordance  with  the 
kind  of  motion  it  produces  or  gives  rise  to  —e.g.,  groups  of  muscles  which 
alternately  bend  or  straighten  a  joint  or  alternately  diminish  or  increase 
the  angular  distance  between  two  bones,  are  known,  respectively,  as 
flexors  and  extensors;  such  muscle  groups  are  in  association  with  ginglymus 
joints.  Muscles  which  turn  the  bone  to  which  they  are  attached  around 
its  own  axis  without  producing  any  great  change  of  position  are  known  as 
rotators,  and  are  in  association  with  the  enarthrodial  or  ball-and-socket 
joints.  Muscles  which  impart  an  angular  movement  of  the  extremities 
to  and  from  the  median  line  of  the  body  are  termed  abductors  and  adductors. 
In  addition  to  the  actions  of  individual  groups  of  muscles  in  causing 
special  movements  in  some  regions,  several  groups  of  muscles  are  coordi- 
nated for  the  accomplishment  of  certain  definite  functions — e.g.,  muscles 
of  respiration,  mastication,  expression.  The  coordination  of  axial  and 
appendicular  muscles  enables  the  individual  ^ 

to  assume  certain  postures,  such  as  standing      ^ 

and  sitting;  to  perform  various  acts  of  locomo-      y^  ^  p  (l) 

tion,  as  walking,  running,  swimming,  etc. 


• 

F 

1 

w 

F 

A 

p 

J 

A 

9 

W 

1 

p 

Levers. — The  function  or  special  mode  of     y:^ ^ d(^) 

action   of  individual   muscles  can  be  under- 
stood only  when  the  bones  with  which  they      —  ^       a  /«\ 
are  connected  are  regarded  as  levers  whose     ^                     PA 
fulcra  or  fixed  points  lie  in  the  joints  where  the     Fig.  2.— The  Three  Orders 
movement  takes  place,  and  when  the  muscles 

are  considered  as  sources  of  power  for  imparting  movement  to  the  levers, 
with  the  object  of  overcoming  resistance  or  raising  weights. 

In  mechanics,  levers  of  three  kinds  or  orders  are  recognized,  according 
to  the  relative  position  of  the  fulcrum  or  axis  of  motion,  the  applied 
power,  and  the  weight  to  be  moved.     (See  Fig.  2.) 

In  levers  of  the  first  order  the  fulcrum,  F,  lies  between  the  weight  or 
resistance,  W,  and  the  power  of  moving  force,  P.  The  distance  P-F 
is  known  as  the  power  arm,  the  distance  W-F  as  the  weight  arm.  As  an 
example  of  this  form  of  lever  in  the  human  body  may  be  mentioned : 

1.  The  elevation  of  the  trunk  from  the  flexed  position.  The  axis  of 
movement,  the  fulcrum,  lies  in  the  hip-joint;  the  weight,  that  of  the 
trunk,  acting  as  if  concentrated  at  its  center  of  gravity,  lies  between  the 
shoulders;  the  power,  the  contracting  muscles  attached  to  the  tuberosity 
of  the  ischium.  The  opposite  movement  is  equally  one  of  the  first  order, 
but  the  relative  positions  of  P  and  W  are  reversed. 

2.  The  skull  in  its  movements  backward  and  forward  upon  the  atlas. 


30  HUMAN  PHYSIOLOGY 

In  levers  of  the  second  order  the  weight  lies  between  the  power  and  the 
fulcrum.    As  an  illustration  of  this  form  of  lever  may  be  mentioned: 

1.  The  depression  of  the  lower  jaw,  in  which  movement  the  fulcrum 
is  the  temporomaxillary  articulation;  the  resistance,  the  tension  of  the 
elevator  muscles;  the  power,  the  contraction  of  the  depressor  muscles. 

2.  The  raising  of  the  body  on  the  toes — F  being  the  toes,  W  the  weight 
of  the  body  acting  through  the  ankle,  P  the  gastrocnemius  muscle  acting 
upon  the  heel  bone. 

In  levers  of  the  third  order  the  power  is  applied  at  a  point  lying  between 
the  fulcrum  and  the  weight.  As  examples  of  this  form  of  lever  may  be 
mentioned : 

1.  The  flexion  of  the  forearm — F  being  the  elbow-joint,  P  the  contract- 
ing biceps  and  brachialis  anticus  muscles  applied  at  their  insertion,  W 
the  weight  of  the  forearm  and  hand, 

2.  The  extension  of  the  leg  on  the  thigh. 

When  levers  are  employed  in  mechanics,  the  object  aimed  at  is  the 
overcoming  of  a  great  resistance  by  the  application  of  a  small  force  acting 
through  a  great  distance,  so  as  to  obtain  a  mechanical  advantage.  In  the 
mechanism  of  the  human  body  the  reverse  generally  obtains — viz.,  the 
overcoming  of  a  small  resistance  by  the  application  of  a  great  force  acting 
through  a  small  distance.  As  a  result,  there  is  a  gain  in  the  extent  and 
rapidity  of  movement  of  the  lever.  The  power,  however,  owing  to  its 
point  of  application,  acts  at  a  great  mechanical  disadvantage  in  many 
instances,  especially  in  levers  of  the  third  order. 

Postures. — Owing  to  its  system  of  joints,  levers,  and  muscles,  the 
human  body  can  assume  a  series  of  positions  of  equilibrium,  such  as  stand- 
ing, and  sitting,  to  which  the  name  posture  has  been  given.  In  order 
that  the  body  may  remain  in  a  state  of  stable  equilibrium  in  any  posture, 
it  is  essential  that  the  vertical  line  passing  through  the  center  of  gravity 
shall  fall  within  the  base  of  support. 

Standing  is  that  position  of  equilibrium  in  which  a  line  drawn  through 
the  center  of  gravity  falls  within  the  area  of  both  feet  placed  on  the 
ground.     This  position  is  maintained: 

1.  By  firmly  fixing  the  head  on  top  of  the  vertebral  column  by  the 
action  of  the  muscles  on  the  back  of  the  neck. 

2.  By  making  the  vertebral  column  rigid,  which  is  accomplished  by  the 
longissimus  dorsi  and  the  quadratus  lumborum  muscles.  This  having 
been  accomplished,  the  center  of  gravity  falls  in  front  of  the  tenth  dorsal 
vertebra;  the  vertical  line  passing  through  this  point  falls  behind  the  line 
connecting  both  hip-joints.     In  consequence,  the  trunk  is  not  balanced 


SPECIAL   PHYSIOLOGY   OF   MUSCLES  3 1 

on  the  hip-joints,  and  would  fall  backward  were  it  not  prevented  by  the 
contraction  of  the  rectus  femoris  muscle  and  ligaments.  At  the  knees 
and  ankles  a  similar  balancing  of  the  parts  bove  is  brought  about  by  the 
action  of  various  muscles.  When  the  entire  body  is  in  the  erect  or 
military  position,  the  arms  by  the  sides,  the  center  of  gravity  lies  between 
the  sacrum  and  the  last  lumbar  vertebra,  and  the  vertical  line  touches  the 
ground  between  the  feet  and  within  the  base  of  support. 

Sitting  erect  is  a  condition  of  equilibrium  in  which  the  body  is  balanced 
on  the  tubera  ischii,  when  the  trunk  and  head  together  form  a  rigid  column. 
The  vertical  line  passes  between  the  tubera. 

Locomotion  is  the  act  of  transferring  the  body,  as  a  whole,  through  space, 
and  is  accomplished  by  the  combined  action  of  its  own  muscles.  The 
acts  involved  consist  of  walking,  running,  jumping,  etc. 

Walking  is  a  complicated  act,  involving  almost  all  the  voluntary  muscles 
of  the  body,  either  for  purposes  of  progression  or  for  balancing  the  head 
and  trunk,  and  may  be  defined  as  a  progression  in  a  forward  horizontal 
direction,  due  to  the  alternate  action  of  both  legs.  In  walking,  one  leg 
becomes  for  the  time  being,  the  active  or  supporting  leg,  carrying  the 
trunk  and  head;  the  other,  the  passive  but  progressive  leg,  to  become  in 
turn  the  active  leg  when  the  foot  touches  the  ground.  Each  leg,  therefore, 
is  alternately  in  an  active  and  a  passive  state. 

Running  is  distinguished  from  walking  by  the  fact  that,  at  a  given 
moment,  both  feet  are  off  the  ground  and  the  body  is  raised  in  the  air. 

While  the  limits  of  a  compend  do  not  permit  of  a  description  of  the 
origin,  insertion,  and  mode  of  action  of  the  individual  muscles  of  the 
body,  it  has  been  thought  desirable  to  call  attention  to  a  few  of  the 
principal  muscles  whose  function  it  is  to  produce  special  forms  of  move- 
ment, as  well  as  locomotion  (See  Fig.  3).  The  erect  position  is  largely 
maintained  by  the  fixation  of  the  spinal  column  and  the  balancing  of 
the  head  upon  its  upper  extremity;  the  former  is  accompanied  by  the 
erector  spince  muscle,  named  from  its  function  and  its  fleshy  continuations, 
situated  on 'each  side  of  the  vertebral  column.  Arising  from  the  pelvis 
and  lumbar  vertebrae,  this  muscle  passes  upward,  and  is  attached  by  its 
continuations  to  all  the  vertebrae.  Its  action  is  to  extend  the  vertebral 
column  and  to  maintain  the  erect  position.  The  head  is  balanced  upon 
the  top  of  the  vertebral  column  by  the  combined  action  o^  the  trapezius 
and  suboccipital  muscles  forming  the  nape  of  the  neck,  and  by  the  sterno- 
cleido-mastoid  muscle.  This  latter  muscle  arises  from  the  inner  third  of 
the  clavicle  and  upper  border  of  the  sternum.  It  is  inserted  into  the 
temporal  bone  just  behind  the  ear.    Its  action  is  to  flex  the  head  laterally 


32 


HUMAN  PHYSIOLOGY 


Fig.  3. — Superficial  Muscles  of  the  Body. 


SPECIAL  PHYSIOLOGY   OF   MUSCLES  33 

and  to  rotate  the  face  to  the  opposite  side.     When  both  muscles  act 
simultaneously,  the  head  and  neck  are  flexed  upon  the  thorax. 

The  temporal  and  masseter  muscles,  situated  at  the  side  of  the  head, 
arise  respectively  from  the  temporal  fossa  and  the  zygomatic  arch,  and 
are  inserted  into  the  ramus  of  the  lower  jaw.  Their  action  is  to  close  the 
mouth  and  to  assist  in  mastication.  The  occipito-frontalis,  the  orbicularis 
palpebrarum,  and  orbicularis  oris  muscles  are  largely  concerned  in  wrink- 
ling the  forehead,  closing  the  eyes  and  mouth,  and  in  giving  various 
expressions  to  the  face. 

The  deltoid  is  a  thick,  triangular  muscle  covering  the  shoulder-joint. 
Arising  from  ths  outer  third  of  the  clavicle,  the  acromial  process,  and  the 
spine  of  the  scapula,  its  fibers  converge  to  be  inserted  into  the  humerus 
just  above  its  middle  point.  Its  action  is  to  elevate  the  arm  through  a 
right  angle.  Owing  to  its  point  of  insertion  it  acts  as  a  lever  of  the  third 
order,  but,  notwithstanding  the  advantageous  points  of  insertion,  it 
acts  at  a  considerable  disadvantage,  owing  to  the  obliquity  of  its  direction. 

The  biceps  muscle,  situated  on  the  anterior  aspect  of  the  arm,  arises 
from  the  upper  border  of  the  glenoid  fossa  and  the  coracoid  process,  and 
is  inserted  into  the  radius  just  beyond  the  elbow-joint.  Its  action  is  to 
flex  and  supinate  the  forearm  and  to  place  it  in  the  most  favorable  position 
for  striking  a  blow.  When  the  forearm  is  fixed,  it  assists  in  flexing  the 
arm,  as  in  climbing. 

The  triceps  muscle,  situated  on  the  back  of  the  arm,  arises  from  the 
scapula  and  the  posterior  surface  of  the  humenis,  and  is  inserted  in  the 
olecranon  process  of  the  ulna.  In  its  action  it  directly  antagonizes  the 
biceps,  namely,  extending  the  forearm.  In  so  doing  it  acts  as  a  lever  of 
the  first  order.  The  short  distance  between  the  muscular  insertion  and 
the  fulcrum  causes  it  to  act  at  a  great  mechanical  disadvantage,  but  there 
is  a  corresponding  gain  in  both  speed  and  range  of  movement.  The 
muscles  of  the  forearm  are  very  numerous.  Their  action  is  to  impart 
to  the  forearm  and  hand  a  variety  of  movements,  such  as  pronation, 
supination,  flexion,  extension,  rotation,  etc. 

The  pectoralis  major  and  pectoralis  minor  muscles  form  the  fleshy  masses 
of  the  breast.  Arising  from  the  inner  half  of  the  clavicle,  the  side  of  the 
sternum,  and  the  outer  surfaces  of  the  third,  fourth,  and  fifth  ribs  an- 
teriorly, the  muscle-fibers  converge  to  be  inserted  into  the  humerus  and 
coracoid  process.  Their  combined  action  is  to  adduct,  flex  and  rotate 
the  arm  inward  and  to  draw  the  scapula  downward  and  forward,  move- 
ments necessary  to  the  folding  of  the  arms  across  the  chest. 

The  rectus  abdominis  and  the  obliquus  externus  assist  in  forming  the 
abdominal  walls. 
3 


34  HUMAN   PHYSIOLOGY 

The  glutei  muscles  are  three  in  number,  are  arranged  in  layers,  and 
form  the  fleshy  masses  known  as  the  buttocks.  They  arise  from  the  side 
of  the  pelvis  and  are  attached  to  the  femur  in  the  neighborhood  of  the 
great  trochanter.  Their  action  is  to  extend  the  hips,  to  raise  the  body 
from  the  stooping  position,  and  to  assist  in  walking  by  firmly  holding  the 
pelvis  on  the  thigh  while  the  opposite  leg  is  advanced  in  the  forward 
direction. 

The  rectus  femoris,  with  its  associates,  the  vastus  internus  and  vastus 
externus  and  the  crureus,  forms  the  fleshy  mass  on  the  anterior  surface  of 
the  thigh.  The  former  arises  from  the  anterior  part  of  the  ilium,  the 
latter  from  the  femur.  Their  common  tendon,  which  is  united  to  the 
patella,  is  continued  as  the  ligamentum  patellae,  which  is  attached  to  the 
upper  part  of  the  tibia.  The  action  of  this  muscular  group  is  to  extend 
the  leg,  to  flex  the  thigh,  and  to  raise  the  entire  weight  of  the  body,  as 
in  changing  from  the  sitting  to  the  erect  position. 

The  hiceps  femoris  muscle,  situated  on  the  outer  and  posterior  aspect 
of  the  thigh,  arises  from  the  tuber  ischii,  and  is  inserted  into  the  head  of 
the  fibula. 

The  semimembranosus  and  the  semitendinosus  muscles,  situated  on  the 
inner  and  posterior  aspect  of  the  thigh,  are  inserted  into  the  head  of  the 
tibia.  Their  combined  action  is  to  extend  the  hips  and  to  flex  the  knee. 
Acting  from  below,  they  assist  in  raising  the  body  from  the  stooping 
position. 

The  gastrocnemius  muscle  forms  the  enlargement  known  as  the  calf  of 
the  leg.  It  arises  by  two  heads  from  the  condyles  of  the  femur.  Its 
tendon,  the  tendo  Achillis,  is  inserted  into  the  posterior  surface  of  the 
heel  bone.  Its  action  is  to  extend  the  foot  and  to  raise  the  weight  of  the 
body  in  walking  and  running.  On  the  front  of  the  leg  are  numerous 
muscles — e.g.,  tibialis  anticus,  peroneus  longus,  etc.,  the  action  of  which 
is  to. flex  the  foot  and  to  antagonize  the  gastrocnemius. 

PHYSIOLOGY  OF  NERVE  TISSUE 

The  nerve  tissue,  which  unites  and  coordinates  the  various  organs 
and  tissues  of  the  body  and  brings  the  individual  into  relationship  with 
the  external  world,  is  arranged  anatomically  into  two  systems,  termed  the 
encephalo  or  cerebrospinal  and  the  sympathetic. 

The  encephalo-spinal  or  cerebro -spinal  system  consists  of: 

I.  The  brain  and  spinal  cord,  contained  within  the  cavities  of  the 
cranium  and  the  spinal  column  respectively,  and 


PHYSIOLOGY  OF  NERVE  TISSUE  35 

2.  The  cranial  and  spinal  nerves. 
The  sympathetic  system  consists  of: 

1.  A  double  chain  of  ganglia  situated  on  each  side  of  the  spinal  column 
and  extending  from  the  base  of  the  skull  to  the  tip  of  the  coccyx. 

2.  Various  collections  of  ganglia  situated  in  the  head,  face,  thorax, 
abdomen,  and  pelvis.  All  these  ganglia  are  united  by  an  elaborate 
system  of  intercommunicating  nerves,  many  of  which  are  connected 
with  the  cerebro-spinal  system. 

HISTOLOGY  OF  NERVE  TISSUE 

The  Neuron. — The  nerve  tissue  has  been  resolved  by  the  investigations 
of  modern  histologists  into  a  single  morphologic  unit,  to  which  the  term 
neuron  has  been  applied.  The  entire  nervous  system  has  been  shown 
to  be  but  an  aggregate  of  an  infinite  number  of  neurons,  each  of  which 
is  histologically  distinct  and  independent.  Though  having  a  common 
origin,  as  shown  by  embryologic  investigations,  they  have  acquired  a 
variety  of  forms  in  different  parts  of  the  nervous  system  in  the  course 
of  development.  The  old  conception  that  the  nervous  system  consists 
of  two  distinct  histologic  elements,  nerve-cells  and  nerve-fibers,  which 
differed  not  only  in  their  mode  of  origin,  but  also  in  their  properties,  their 
relation  to  each  other,  and  their  functions,  has  been  entirely  disproved. 

The  neuron,  or  neurologic  unit,  is  histologically  a  nerve-cell,  the  surface 
of  which  presents  a  greater  or  less  number  of  processes  in  varying  degrees 
of  differentiation.  As  represented  in  figure  7,  the  neuron  may  be  said 
to  consist  of:  (i)  The  nerve-cell,  neurocyte,  or  corpus;  (2)  the  axon,  or 
nerve  process;  (3)  the  end  tufts,  or  terminal  branches!  Though  these 
three  main  histologic  features  are  everywhere  recognizable,  they  exhibit 
a  variety  of  secondary  features  in  different  situations  in  accordance  with 
peculiarities  of  function.  Though  the  nerve-cell  and  the  nerve-fiber  are 
but  part  of  the  same  neuron,  it  is  convenient  at  present  to  describe  them 
separately. 

The  Nerve-cell. — The  nerve-cell,  or  body  of  the  neuron,  presents  a 
variety  of  shapes  and  sizes  in  different  portions  of  the  nervous  system. 
Originally  ovoid  in  shape,  it  has  acquired,  in  course  of  development, 
peculiarities  of  form  which  are  described  as  pyramidal,  stellate,  pear- 
shaped,  spindle-shaped,  etc.  The  size  of  the  cell  varies  considerably, 
the  smallest  having  a  diameter  of  not  more  than  Mooo  of  an  inch,  the 
largest  not  more  than  J^oo  of  an  inch.    Each  cell  consists  of  granular 


36 


HUMAN    PHYSIOLOGY 


striated  protoplasm,  containing  a  distinct  vesicular  nucleus  and  a  well- 
defined  nucleolus.  A  cell  membrane  has  not  been  observed.  From  the 
surface  of  the  adult  cell  portions  of  the  protoplasm  are  projected  in  various 
directions,  which  portions,  rapidly  dividing  and  subdividing,  form  a  series 
of  branches,  termed  dendrites  or  dendrons.  In  some  situations  the  ulti- 
mate branches  of  the  dendrites  present  short  lateral  processes,  known  as 
lateral  buds,  or  gemmiiles,  which  impart  to  the  branches  a  feathery  appear- 
ance.    This  characteristic  is  common  to  the  cells  of  the  cortex,  of  the 


Fig.  4. — A.  Efferent  Neuron;  B,  Afferent  Neuron.     Pound  in  both  Spinal 
AND  Cranial  Nerves. 


cerebrum,  and  of  the  cerebellum.  The  ultimate  branches  of  the  dendrites, 
though  forming  an  intricate  feltwork,  never  anastomose  with  one  another, 
nor  unite  with  dendrites  of  adjoining  cells.  According  to  the  number  of 
axons,  nerve-cells  are  classified  as  monaxonic,  diaxonic,  polyaxonic 
Most  of  the  cells  of  the  nervous  system  of  the  higher  vertebrates  are 
monaxonic.  In  the  ganglia  of  the  posterior  or  dorsal  roots  of  the  spinal 
and  cranial  nerves,  however,  they  are  diaxonic.  In  this  situation  the 
axons,  emerging  from  opposite  poles  of  the  cell,  either  remain  separate 
and  pursue  opposite  directions,  or  unite  to  form  a  common  stem,  which 


PHYSIOLOGY   OF   NERVE   TISSUE  37 

subsequently  divides  into  two  branches,  which  then  pursue  opposite  direc- 
tions. (See  Fig.  4.)  The  nerve-cell  maintains  its  own  nutrition,  and 
presides  over  that  of  the  dendrites  and  the  axon  as  well.  If  the  latter 
be  separated  in  any  part  of  its  course  from  the  cell,  it  speedily  degenerates 
and  dies. 

The  axon,  or  nerve  process,  arises  from  a  cone-shaped  projection  from  the 
surface  of  the  cell,  and  is  the  first  outgrowth  from  its  protoplasm.  At  a 
'  short  distance  from  its  origin  it  becomes  markedly  differentiated  from  the 
dendrites  which  subsequently  develop.  It  is  characterized  by  a  sharp, 
regular  outline,  a  uniform  diameter,  and  a  hyaline  appearance.  In 
structure,  the  axon  appears  to  consist  of  fine  fibrillae  embedded  in  a  clear, 
protoplasmic  substance.  Shafer  advocates  the  view  that  the  fibrillae  are 
exceedingly  fine  tubes  filled  with  fluid.  The  axon  varies  in  length  from  a 
few  millimeters  to  100  cm.  In  the  former  instance  the  axon,  at  a  short 
distance  from  its  origin,  divides  into  a  number  of  branches,  which  form  an 
intricate  feltwork  in  the  neighborhood  of  the  cell.  In  the  latter  instance 
the  axon  continues  for  an  indefinite  distance  as  an  individual  structure. 
In  its  course,  however,  especially  in  the  central  nervous  system,  it  gives 
off  a  number  of  collateral  branches,  which  possess  all  its  histologic  features. 
The  long  axons  seem  to  bring  the  body  of  the  cell  into  direct  relation  with 
peripheral  organs,  or  with  more  or  less  remote  portions  of  the  nervous 
system,  thus  constituting  association  or  commissural  fibers. 

The  more  or  less  elongated  axon  becomes  invested,  as  a  rule,  at  a  short 
distance  from  the  cell  with  nucleated  oblong  cells,  which  subsequently  be- 
come modified  and  constitute  a  medullary  or  myelin  sheath.  This  is  in- 
vested by  a  thin,  cellular  membrane — the  neurilemma.  These  three  struc- 
tures thus  constitute  what  is  known  as  a  meduUated  nerve-fiber.  In  the 
central  nervous  system  the  outer  sheath  is  frequently  absent.  In  the 
sympathetic  system  the  myelin  is  frequently  absent,  though  the  axon  is 
inclosed  by  the  neurilemma,  thus  constituting  a  non-meduUated  nerve- 
fiber. 

The  end  tufts  or  terminal  organs  are  formed  by  the  splitting  of  the  axon 
into  a  number  of  filaments,  which  remain  independent  of  one  another  and 
are  free  from  the  medullary  investment.  The  histologic  peculiarities  of 
the  terminal  organs  vary  in  different  situations,  and  in  many  instances  are 
quite  complex  and  characteristic.  In  peripheral  organs,  as  muscles, 
glands,  blood-vessels,  skin,  mucous  membrane,  the  tufts  are  in  direct 
organic  connection  with  their  cellular  elements.  In  the  central  nerve 
system  the  tufts  are  in  more  or  less  intimate  relation  with  the  dendrites  of 
adjacent  neurons. 

The  neurons  in  their  totality  constitute  the  neuron  or  nerve  tissue. 


38  HUMAN  PHYSIOLOGY 

From  the  fact  that  they  are  arranged  both  serially  and  collaterally  into 
a  regular  and  connected  whole,  they  collectively  constitute  the  system 
known  as  the  neuron  or  nerve  system.  The  neurons  moreover  are  grouped 
into  more  or  less  complexly  organized  masses  termed  organs  which  in 
accordance  with  their  actions  may  be  divided  for  convenience  into  central 
and  peripheral  organs. 

The  Central  Organs  of  the  Nerve  System. — The  central  organs  con- 
sist of  the  encephalon  and  spinal  cord,  contained  within  the  cavities  of  the 
head  and  spinal  column  respectively.  They  consist  of  neurons  arranged 
in  a  very  complex  manner.  In  a  subsequent  chapter  the  anatomic 
arrangement  of  their  constituent  parts  will  be  detailed. 

The  Peripheral  Organs  of  the  Nerve  System. — These  consist  of  the 
cranial  and  spinal  nerves  and  the  sympathetic  ganglia.  Each  nerve 
consists  of  a  variable  number  of  neurons  united  into  firm  bundles  by  con- 


FiG.  5. — Transverse  Section  of  a  Nerve  (Median). 
ep.  Epineurium.     pe.  Perineurium,     ed.  Endoneurium. — {Landois  and  Stirling.) 

nective  tissue  which  supports  blood-vessels  and  lymphatics.    The  bundles 
are  technically  known  as  nerve-trunks  or  nerves. 

The  nerve-trunks  connect  the  brain  and  cord  with  all  the  remaining 
structures  of  the  body.  Each  nerve  is  invested  by  a  thick  layer  of  lamel- 
lated  connective  tissue,  known  as  the  epineurium.  A  transverse  section 
of  a  nerve  shows  (see  Fig.  5),  that  it  is  made  up  of  a  number  of  small 
bundles  of  fibers  each  of  which  possesses  a  separate  investment  of  con- 
nective tissue — the  perineurium.  Within  this  membrane  the  nerve-fibers 
are  supported  by  a  fine  stroma — the  endoneurium.  After  pursuing  a 
longer  or  shorter  course,  the  nerve  trunk  gives  off  branches,  which  inter- 
lace very  freely  with  neighboring  branches,  forming  plexuses,  the  fibers 
of  which  are  distributed  to  associated  organs  and  regions  of  the  body. 


PHYSIOLOGY   OF   NERVE   TISSUE 


39 


From  their  origin  to  their  termination,  however,  nerve-fibers  retain  their 
individuality,  and  never  become  blended  with  adjoining  fibers. 

As  nerves  pass  from  their  origin  to  their  peripheral  terminations,  they 
give  off  a  number  of  branches,  each  of  which  becomes  invested  with  a 
lamellated  sheath — an  offshoot  from  that  investing  the  parent  trunk. 
This  division  of  nerve  bundles  and  sheath  continues  throughout  all  the 
branches  down  to  the  ultimate  nerve-fibers,  each  of  which  is  surrounded 
by  a  sheath  of  its  own,  consisting  of  a  single  layer  of  endothelial  cells. 
This  delicate  transparent  membrane,  the  sheath  of  Henle,  is  separated 
from  the  nerve-fiber  by  a  considerable  space,  in  which  is  contained  lymph 
destined  for  the  nutrition  of  the  fiber.  Near  their  ultimate  terminations 
the  nerve-fibers  themselves  undergo  division,  so  that  a  single  fiber  may 
give  origin  to  a  number  of  branches,  each  of  which  contains  a  portion  of 
the  parent  axis-cylinder  and  myelin. 

The  neurons  composing  the  spinal  and  cranial  nerves  are  represented  in 
Fig.  6,  which  are  connected  peripherally  by  their  terminal  branches  with 


Fig.  6. — Diagram  of  a  Simple  Reflex  Arc. 

I.  Sentient  surface.     2.  Afferent  nerve.     3.  Emissive  or  motor  cell.     4.  Efferent 

nerve.     S-  Muscle.— (After  Moral  and  Day  on.) 

muscles  on  the  one  hand  and  with  epithelium  of  skin,  mucous  membrane 
etc.,  on  the  other  hand.  In  the  spinal  cord  the  terminal  branches  of  the 
afferent  neuron  come  into  histologic  and  physiologic  relation  with  the 
dendrites  of  a  second  neuron,  the  axonic  process  of  which  in  many  instances 
ascends  the  cord  to  different  levels  or  even  as  far  as  the  brain,  where  its 
terminal  branches  come  into  relation  with  the  dendrites  of  still  another 
neuron,  the  axonic  process  of  which  is  in  turn  connected  with  neurons  in 
the  cortex  of  either  the  cerebrum  or  cerebellum.  The  surfaces  of  the  body 
are  thus  brought  into  relation  with  the  cerebral  and  cerebellar  neurons. 
The  neurons  arranged  in  this  serial  manner  constitute  the  aferent  side  of 
the  nerve  system. 


4©  HUMAN   PHYSIOLOGY 

In  a  similar  way  the  efferent  neurons  o^  the  spinal  and  cranial  nerves  are 
brought  into  relation  with  the  cortex  of  the  cerebrum.  Large  pyramidal- 
shaped  neurocytes  situated  in  specialized  regions  of  the  cortex  of  the  cere- 
brum send  their  axonic  processes  down  through  the  brain  and  cord.  As 
they  approach  their  destination  the  terminal  branches  become  related 
histologically  and  physiologically  with  the  dendrites  of  the  neurons  com- 
posing the  cranial  and  spinal  nerves.  The  cortex  of  the  cerebrum  is  thus 
brought  into  relation  with  the  general  musculature  of  the  body.  The 
neurones  arranged  in  this  serial  maner  constitute  the  efferent  side  of 
the  nerve  system. 

Sympathetic  Ganglia. — A  sympathetic  ganglion  consists  essentially  of 
a  connective-tissue  capsule  with  an  interior  framework.  The  meshes  of 
this  framework  contain  nerve-cells  possessing  dendrites  and  branching 
axons.  The  majority  of  the  axons  are  devoid  of  myelin  and  are  therefore 
known  as  non-myehneated  nerve-fibers.  Owing  to  the  absence  of  the 
myelin  they  present  a  rather  pale  or  grayish  appearance.  In  all  instances, 
with  the  exception  of  the  ganglion  cells  of  the  heart,  the  axons  are  dis- 
tributed to  non-striated  muscle  tissue  and  to  the  epithelium  of  glands. 

The  nerve-cells  of  the  ganglia  are  also  in  histologic  connection  with  the 
terminal  branches  of  certain  fine  meduUated  nerve-fibers  which  leave  the 
spinal  cord  by  way  of  the  ventral  roots  of  the  spinal  nerves.  These  nerve- 
fibers  are  designated  pre- ganglionic  fibers,  while  those  emerging  from  the 
cells  are  designated  post-gan glionic  fibers. 

THE  RELATON  OF  THE  PERIPHERAL  ORGANS  OF  THE 
NERVE  SYSTEM  TO  THE  CENTRAL  ORGANS 

Spinal  Nerves. — The  nerves  in  connection  with  the  spinal  cord  are 
thirty-one  in  number  on  each  side.  If  traced  toward  the  spinal  column, 
it  will  be  found  that  the  nerve-trunk  passes  through  an  intervertebral 
foramen.  Near  the  outer  limits  of  the  foramina  each  nerve-trunk  divides 
into  two  branches,  generally  termed  roots,  one  of  which,  curving  slightly 
forward  and  upward,  enters  the  spinal  cord  on  its  anterior  or  ventral  sur- 
face, while  the  other,  curving  backward  and  upward,  enters  the  spinal 
cord  on  its  posterior  or  dorsal  surface.  The  former  is  termed  the  anterior 
or  ventral  root;  the  latter,  the  posterior  or  dorsal  root.  Each  dorsal  root 
presents  near  its  union  with  the  ventral  root  a  small  ovoid  grayish  enlarge- 
ment known  as  a  ganglion.  Both  roots  previous  to  entering  the  cord 
subdivide  into  from  four  to  six  fasciculi. 

A  microscopic  examination  of  a  cross-section  of  the  spinal  cord  shows 
that  the  fibers  of  the  ventral  roots  can  be  traced  directly  into  the  body  of 


PHYSIOLOGY    OF    NERVE   TISSUE  4 1 

the  nerve-cells  in  the  ventral  horns  of  the  gray  matter.  The  fibers  of  the 
dorsal  roots  are  not  so  easily  traced,  for  they  diverge  in  several  directions 
shortly  after  entering  the  cord.  In  their  course  they  give  off  collateral 
branches  which,  in  common  with  the  main  fiber,  and  in  tufts  which  become 
associated  with  nerve-cells  in  both  the  ventral  and  dorsal  horns  of  the 
gray  matter. 

Cranial  Nerves. — The  nerves  in  connection  with  the  base  of  the  brain 
are  known  as  cranial  nerves;  some  of  these  nerves  present  a  similar  gan- 
glionic enlargement,  and  therefore  may  be  regarded  as  dorsal  nerves,  while 
others  may  be  regarded  as  ventral  nerves.  Their  relations  within  the 
medulla  oblongata  are  similar  to  those  within  the  spinal  cord. 

Development  and  Nutrition  of  Nerves. — The  efferent  nerve-fibers, 

which  constitute  some  of  the  cranial  nerves  and  all  the  ventral  roots  of 
the  spinal  nerves,  have  their  origin  in  cells  located  in  the  gray  matter 
beneath  the  aqueduct  of  Sylvius,  beneath  the  floor  of  the  fourth  ventricle 
and  in  the  anterior  horns  of  the  gray  matter  of  the  spinal  cord.  These 
cells  are  the  modified  descendants  of  independent,  oval,  pear-shaped 
cells — the  neuroblasts — which  migrate  from  the  medullary  tube.  As 
they  approach  the  surface  of  the  cord  their  axons  are  directed  toward 
the  ventral  surface,  which  eventually  they  pierce.  Emerging  from  the 
cord,  the  axons  continue  to  grow,  and  become  invested  with  the  myehn 
sheath  and  neurilemma,  thus  constituting  the  ventral  roots. 

The  afferent  nerve-fibers,  which  constitute  some  of  the  cranial  nerves 
and  all  the  dorsal  roots  of  the  spinal  nerves,  develop  outside  of  the 
central  nervous  system  and  only  subsequei-Jy  become  connected  with 
it.  At  the  time  of  the  closure  of  the  medullary  tube  a  band  or  ridge 
of  epithelial  tissue  develops  near  the  dorsal  surface,  which,  becoming 
segmented,  moves  outward  and  forms  the  rudimentary  spinal  ganglia. 
The  cells  in  this  situation  develop  two  axons,  one  from  each  end  of  the 
cell,  which  pass  in  opposite  directions,  one  toward  the  spinal  cord,  the 
other  toward  the  periphery.  In  the  adult  condition  the  two  axons  shift 
their  position,  unite,  and  form  a  T-shaped  process,  after  which  a  division 
into  two  branches  again  takes  place.  In  the  ganglia  of  all  the  sensori- 
cranial  and  sensorispinal  nerves  the  cells  have  this  histologic  peculiarity. 

Efferent  and  Afferent  Nerves. — Nerves  are  channels  of  communica- 
tion between  the  brain  and  spinal  cord,  on  the  one  hand,  and  the  skeletal 
muscles,  glands,  blood-vessels,  visceral  muscles,  skin,  mucous  membrane, 
etc.,  on  the  other.  Some  of  the  nerve-fibers  serve  for  the  transmission  of 
nerve  energy  from  the  brain  and  spinal  cord  to  certain  peripheral  organs, 


42  HUMAN  PHYSIOLOGY 

and  so  accelerate  or  retard,  augment  or  inhibit  their  activities;  others 
serve  for  the  transmission  of  nerve  energy  from  certain  peripheral  organs 
to  the  brain  and  spinal  cord  which  gives  rise  to  sensation  or  other  modes 
of  nerve  activity.  The  former  are  termed  efferent  or  centrifugal,  the  latter 
afferent  or  centripetal  nerves.  Experimentally  it  has  been  determined 
that  the  anterior  or  ventral  roots  contain  all  the  efferent  fibers,  the  posterior 
or  dorsal  roots  all  the  afferent  fibers. 

The  efferent  nerves  may  be  classified,  in  accordance  with  their  dis- 
tribution and  the  characteristic  forms  of  activity  to  which  they  give  rise, 
into  several  groups,  as  follows: 

1.  Skeletal-muscle  or  motor  nerves,  those  which  convey  nerve  energy  or 
nerve  impulses  directly  to  skeletal-muscles  and  excite  them  to  activity. 

2.  Gland  or  secretor  nerves,  those  which  convey  nerve  impulses  to  glands 
by  way  of  ganglia  and  influence  in  one  direction  or  another  the  degree  of 
their  activity.  Those  which  cause  the  formation  and  discharge  of  the 
secretion  peculiar  to  the  gland  are  known  as  secre to-motor,  while  those 
which  decrease  or  inhibit  the  secretion  are  known  as  secreto-inhibitor 
nerves. 

3.  Vascular  or  vaso-motor  nerves,  those  which  convey  nerve  impulses 
to  the  muscle-fibers  of  the  blood-vessels  and  change  in  one  direction  or 
the  other  the  degree  of  their  natural  contraction.  Those  which  increase 
the  contraction  are  known  as  vaso-constrictors  or  vaso-augmentors;  those 
which  decrease  the  contraction  are  known  as  vaso-dilatators  or  vaso- 
inhibitors.  The  nerves  which  pass  to  that  specialized  part  of  the  vascular 
apparatus,  the  heart,  transmit  nerve  impulses  which  on  the  one  hand 
accelerate  its  rate  or  augment  its  force,  and  on  the  other  hand  inhibit 
or  retard  its  rate  and  diminish  its  force.  For  this  reasom  they  are  termed 
cardiac  nerves,  one  set  oi  which  is  known  as  cardio-accelerator  and  cardio- 
augmentor,  the  other  as  cardio-inhibitor  nerves. 

4.  Visceral  or  viscero-motor  nerves,  those  which  transmit  nerve  impulses 
to  the  muscle  walls  of  the  viscera  and  change  in  one  direction  or  another 
the  degree  of  their  contraction.  Those  which  increase  or  augment  the 
contraction  are  known  as  viscero-augmentor,  while  those  which  decrease 
or  inhibit  the  contraction  are  known  as  viscero-inhibitor  nerves. 

5.  Hair  bulb  or  pilo-motor  nerves,  those  which  transmit  nerve  impulses 
to  the  muscle-fibers  which  cause  an  erection  of  the  hairs. 

Of  the  foregoing  nerves  the  skeletal-muscle  or  motor  nerves  alone  pass 
directly  to  the  muscle.    The  gland,  the  vascular  and  the  visceral  nerves, 


PHYSIOLOGY   OF   NERVE   TISSUE  43 

all  terminate  at  a  variable  distance  from  the  peripheral  organ  around  a 
local  sympathetic  ganglion,  which  in  turn  is  connected  with  the  peripheral 
organ.  The  former  are  termed  pre-ganglionic,  the  latter  post-ganglionic 
fibers. 

The  afferent  nerves  may  also  be  classified,  in  accordance  with  their 
distribution  and  the  character  of  the  sensations  or  other  modes  of  nerve 
activity  to  which  they  give  rise,  into  several  groups,  as  follows: 

A.  Tegumentary  nerves,  comprising  those  distributed  to  skin,  mucous 
membranes  and  sense  organs  and  which  transmit  nerve  impulses  from 
the  periphery  to  the  nerve  centers.  They  may  be  divided  into  reflex 
and  sensorifacient  nerves. 

1.  Reflex  nerves,  those  which  transmit  nerve  impulses  to  the  spinal 
cord  and  medulla  oblongata,  where  they  give  rise  to  different  modes 
of  nerve  activity.     They  may  be  divided  into : 

a.  Reflex  excitator  nerves,  which  transmit  nerve  impulses  which 
cause  an  excitation  of  nerve  centers  and,  in  consequence,  in- 
creased activity  of  peripheral  organs,  e.g.^  skeletal  muscles, 
glands,  blood-vessels  and  viscera. 

h.  Reflex  inhibitor  nerves,  which  transmit  nerve  impulses  which 
cause  an  inhibition  of  nerve  centers  and,  in  consequence,  de- 
creased activity  of  the  peripheral  organs.  It  is  quite  probable 
that  one  and  the  same  nerve  may  subserve  both  sensation  and 
reflex  action,  owing  to  the  collateral  branches  which  are  given 
off  from  the  afferent  roots  as  they  ascend  the  posterior  column 
of  the  cord. 

2.  Sensorifacient  nerves,  those  which  transmit  nerve  impulses  to  the 
brain  where  they  give  rise  to  conscious  sensations.  They  may  be 
sub-divided  into : 

a.  Nerves  of  special  sense — e.g.^  olfactory,  optic,  auditory,  gus- 
tatory, tactile,  thermal,  pain,  pressure — which  give  rise  to  cor- 
respondingly named  sensations. 

b.  Nerves  of  general  sense — e.g.^  the  visceral  afferent  nerves — those 
which  give  rise  normally  to  vague  and  scarcely  perceptible 
sensations,  such  as  the  general  sensations  of  well-being  or  dis- 
comfort, hunger,  thirst,  fatigue,  sex,  want  of  air,  etc. 

B.  Muscle  nerve,  comprising  those  distributed  to  muscles  and  tendons 
and  which  transmit  nerve  ^ impulses  from  muscles  and  tendons  to 
the  brain,  where.they  give  rise  to  the  so-called  muscle  sensations, 
e.g.  J  the  direction  and  the  duration  of  a  movement,  the  resistance 
offered  and  the  posture  of  the  body  or  of  its  individual  parts. 


44  HUMAN   PHYSIOLOGY 

The  foregoing  classification  of  the  efferent  and  afferent  nerve-fibers 
has  been  established  partly  by  experiment  and  partly  by  histologic 
investigations,  e.g. 

Stimulation  of  the  ventral,  efferent  rodt  fibers  produces: 

1.  Tetanic  contraction  of  skeletal  muscles. 

2.  Discharge  of  secretions  from  glands. 

3.  Increase  in  the  degree  of  the  contraction,  the  tonus,  of  the  muscle 
walls  of  the  peripheral  arteries. 

4.  Variations  in  the  degree  of  the  contraction,  the  tonus,  of  the  muscle 
walls  of  certain  viscera  either  in  the  way  of  augmentation  or  inhibition.^ 

Division  of  the  ventral  root  fibers  is  followed  by: 

1.  Relaxation  of  skeletal  muscles  and  loss  of  movement. 

2.  Cessation  in  the  discharge  of  secretions  from  glands. 

3.  Temporary  dilatation  and  loss  of  the  tonus  of  blood-vessels. 

4.  Temporary  impairment  of  the  normal  activities  of  the  visceral 
muscles  from  loss  of  central  nerve  control;  the  degree  of  impairment 
depending  on  the  nature  of  the  viscus  involved. 

Peripheral  stimulation  of  the  dorsal  afferent  root  fibers  produces : 

1.  Reflex  excitation  of  spinal  centers,  in  consequence  of  which  there  is 
an  increased  activity  of  skeletal  muscles,  glands,  blood-vessels,  and  vis- 
ceral walls. 

2.  Reflex  inhibition  of  spinal  nerve-centers,  in  consequence  of  which 
there  may  be  a  decrease  in  the  activities  of  skeletal  muscles,  glands,  blood- 
vessels, and  viscera. 

3.  Sensations  of  touch,  temperature,  pressure,  and  pain. 

4.  Sensations  of  the  duration  and  direction  of  muscle  movements,  of 
the  resistance  offered  and  of  the  position  of  the  body  or  of  its  individual 
parts  (muscle  sensation). 

Division  of  the  dorsal  root  fibers  is  followed  by : 

1.  Loss  of  the  power  of  exciting  or  inhibiting  reflexly  the  activities  of 
spinal  nerve-centers  and  in  consequence  a  loss  of  the  power  of  exciting  or 
inhibiting  the  activities  of  peripheral  organs. 

2.  Loss  of  sensation  in  all  parts  to  which  they  are  distributed. 

^These  last  three  phenomena  are  especially  associated  with  the  ventral  roots  of 
the  second  thoracic  to  the  third  or  fourth  lumbar  nerves  inclusive. 


PHYSIOLOGY   OF   NERVE   TISSUE  45 

The  ventral  roots  are,  therefore,  efferent  in  functioD,  transmitting  nerve 
impulses  from  the  spinal  cord  to  the  peripheral  organs  which  excite  them 
to  activity. 

The  dorsal  roots  are  afferent  in  function,  transmitting  nerve  impulses 
from  the  general  periphery  to  (a)  the  spinal  cord  where  they  excite  its  con- 
tained nerve-centers  to  activity  or  to  a  more  or  less  complete  cessation  of 
activity  (inhibition),  and  (b)  to  the  cerebrum  where  they  excite  its  centers 
to  activity  with  the  development  of  sensations. 

The  peripheral  terminations  of  the  efferent  nerves  are,  therefore,  to  be 
found  in  close  histologic  relation  with  skeletal-muscle  and  visceral-muscle 
fibers  and  with  gland  epithelium.  The  peculiar  termination  in  each  situa- 
tion has  been  termed  an  "end  organ."  The  afferent  nerves  are  likewise  in 
close  histologic  relation  with  the  skin,  mucous  membrance  and  the  sense 
organs.  The  afferent  end  organs  are  in  some  instances  extremely  complex, 
such  as  those  found  in  the  eye  (retina),  the  internal  ear,  the  nose  and 
tongue. 

The  end  organs  of  the  afferent  nerves  are  specialized,  highly  irritable 
structures  placed  between  the  nerve-fibers  and  the  surface  of  the  body. 
They  are  especially  adapted  for  the  reception  of  those  external  forces 
technically  known  as  stimuli,  and  for  the  liberation  of  energy  capable  of 
exciting  the  nerve-fiber  to  activity. 

Nerve  Degeneration. — If  any  one  of  the  cranial  or  spinal  nerves  be 
divided  in  any  portion  of  its  course,  the  part  in  connection  with  the  peri- 
phery in  a  short  time  exhibits  certain  structural  changes,  to  which  the  term 
degeneration  is  applied.  The  portion  in  connection  with  the  brain  or  cord 
retains  its  normal  condition.  The  degenerative  process  begins  simulta- 
neously throughout  the  entire  course  of  the  nerve,  and  consists  in  a  disin- 
tegration and  reduction  of  the  medulla  and  axis  cylinder  into  nuclei,  drops 
of  myelin,  and  fat,  which  in  time  disappear  through  absorption  leaving 
the  neurilemma  intact.  Coincident  with  these  structural  changes  there 
is  a  progressive  alteration  a.nd  diminution  in  the  excitability  of  the  nerve. 
Inasmuch  as  the  central  portion  of  the  nerve,  which  retains  its  connection 
with  the  nerve-cell,  remains  histologically  normal,  it  has  been  assumed  that 
the  nerve-cells  exert  over  the  entire  course  of  the  nerve-fibers  a  nutritive 
or  a  trophic  influence.  This  idea  has  been  greatly  strengthened  since  the 
discovery  that  the  axis-cylinder,  or  the  axon,  has  its  origin  in  and  is  a 
direct  outgrowth  of  the  cell.  When  separated  from  the  parent  cell,  the 
fiber  appears  to  be  incapable  of  itself  of  maintaining  its  nutrition. 

The  relation  of  the  nerve-cells  to  the  nerve-fibers,  in  reference  to  their 
nutrition,  is  demonstrated  by  the  results  which  follow  section  of  the  ventral 
and  dorsal  roots  of  the  spinal  nerves.     If  the  anterior  root  alone  be  divided, 


46  HUMAN  PHYSIOLOGY 

the  degenerative  process  is  confined  to  the  peripheral  portion,  the  central 
portion  remaining  normal.  If  the  posterior  root  be  divided  on  the  peri- 
pheral side  of  the  ganglion,  degeneration  takes  place  only  in  the  peripheral 
portion  of  the  nerve.  If  the  root  be  divided  between  the  ganglion  and 
the  cord,  degeneration  takes  place  only  in  the  central  portion  of  the 
root.  From  these  facts  it  is  evident  that  the  trophic  centers  for  the 
ventral  and  dorsal  roots  lie  in  the  spinal  cord  and  spinal  nerve  ganglia, 
respectively,  or,  in  other  words,  in  the  cells  of  which  they  are  an  Integra 
part.  The  structural  changes  which  nerves  undergo  after  separation 
from  their  centers  are  degenerative  in  character,  and  the  process  is 
usually  spoken  of,  after  its  discoverer,  as  the  Wallerian  degeneration. 
When  the  degeneration  of  the  efferent  nerves  is  completed,  the  struc- 
tures to  which  they  are  distributed,  especially  the  muscles,  undergo  an 
atrophic  or  fatty  degeneration,  with  a  change  or  loss  of  their  irritability. 
This  is,  apparently,  not  to  be  attributed  merely  to  inactivity,  but  rather  to 
a  loss  of  nerve  influences,  inasmuch  as  inactivity  merely  leads  to  atrophy 
and  not  to  degeneration. 

Reactions  of  Degeneration. — In  consequence  of  the  degeneration  and 
changes  in  irritability  which  occur  in  nerves  when  separated  from  their 
centers  and  in  muscles  when  separated  from  their  related  nerves,  either 
experimentally  or  as  the  result  of  disease,  the  response  of  these  structures 
to  the  induced  and  the  make-and-break  of  the  constant  currents  differs 
from  that  observed  in  the  physiologic  condition.  The  facts  observed 
under  the  application  of  these  two  forms  of  electricity  are  of  the  greatest 
importance  in  the  diagnosis  and  therapeutics  of  the  precedent  lesions. 
The  principal  difference  of  behavior  is  observed  in  the  muscles,  which 
exhibit  a  diminished  or  abolished  excitability  to  the  induced  current,  while 
at  the  same  time  manifesting  an  increased  excitability  to  the  constant 
current;  so  much  so  is  this  the  case  that  a  closing  contraction  is  just  as 
likely  to  occur  at  the  positive  as  at  the  negative  pole.  This  peculiarity 
of  the  muscle  response  is  termed  the  reaction  of  degeneration.  The  syn- 
chronous diminished  excitability  of  the  nerves  is  the  same  for  either  cur- 
rent. The  term  "partial  reaction  of  degeneration"  is  used  when  there  is 
a  normal  reaction  of  the  nerves,  with  the  degenerative  reaction  of  the 
muscles.    This  condition  is  observed  in  progressive  muscular  atrophy. 

PHYSIOLOGIC  PROPERTIES  OF  NERVES 

Nerve  Irritability  or  Excitability  and  Conductivity, — These  terms  are 
employed  to  express  that  condition  of  a  nerve  which  enables  it  to  develop 
and  to  conduct  nerve  impulses  from  the  center  to  the  periphery,  from  the 


PHYSIOLOGY   OF   NERVE   TISSUE  47 

periphery  to  the  center,  in  response  to  the  action  of  stimuli.  A  nerve  is 
said  to  be  excitable  or  irritable  as  long  as  it  possesses  these  capabilities  or 
properties.  For  the  manifestation  of  these  properties  the  nerve  must 
retain  a  state  of  physical  and  chemic  integrity;  it  must  undergo  no  change 
in  structure  or  chemic  composition.  The  irritability  of  an  efferent  nerve 
is  demonstrated  by  the  contraction  of  a  muscle,  by  the  secretion  of  a  gland. 
or  by  a  change  in  the  caliber  of  a  blood-vessel,  whenever  a  corresponding 
nerve  is  stimulated.  The  irritability  of  an  afferent  nerve  is  demonstrated 
by  the  production  of  a  sensation  or  a  reflex  action  when  ever  it  is  stimulated. 
The  irritability  of  nerves  continues  for  a  certin  period  of  time  after  separa- 
tion from  the  nerve  centers  and  even  after  the  death  of  the  animal,  vary- 
ing in  different  classes  of  animals.  In  the  warm-blooded  animals,  in 
which  the  nutritive  changes  take  place  with  great  rapidity,  the  irritability 
soon  disappears — a  result  due  to  disintegrative  changes  in  the  nerve 
caused  by  the  withdrawal  of  the  blood-supply.  In  cold-blooded  animals, 
on  the  contrary,  in  which  the  nutritive  changes  take  place  relatively  slowly, 
the  irritability  lasts,  under  favorable  conditions,  for  a  considerable  time. 
Other  tissues  besides  nerves  possess  irritability,  that  is,  the  property  of 
responding  to  the  action  of  stimuli — e.g.,  glands  and  muscles,  which  re- 
spond by  the  production  of  a  secretion  or  a  contraction. 

Independence  of  Tissue  Irritability. — The  irritability  of  nerves  is 
distinct  and  independent  of  the  irritability  of  muscles  and  glands,  as  shown 
by  the  fact  that  it  persists  in  each  a  variable  length  of  time  after  their 
histologic  connections  have  been  impaired  or  destroyed  by  the  introduction 
of  various  chemic  agents  into  the  circulation.  Curara,  for  example,  in- 
duces a  state  of  complete  paralysis  by  modifying  or  depressing  the  conduc- 
tivity of  the  end  organs  of  the  nerves  just  where  they  come  in  contact  with 
the  muscles  without  impairing  the  irritability  of  either  nerve  trunks  or  mus- 
cles. Atropin  induces  complete  suspension  of  glandular  activity  by  im- 
pairing the  terminal  organs  of  the  secretor  nerves  just  where  they  come 
into  relation  with  the  gland  cells,  without  destroying  the  irritability  of 
either  gland  or  nerve. 

Stimuli  of  Nerves. — Nerves  do  not  possess  the  power  of  spontaneously 
generating  and  propagating  nerve  impulses;  they  can  be  aroused  to  activity 
only  by  the  action  of  an  extraneural  stimulus.  In  the  Uving  condition 
the  stimuli  capable  of  throwing  the  nerve  into  an  active  condition  act 
for  the  most  part  on  either  the  central  or  peripheral  end  of  the  nerve.  In 
the  case  of  motor  nerves  the  stimulus  to  the  excitation,  originating  in  some 
molecular  disturbance  in  the  nerve-cells,  acts  upon  the  nerve-fibers  in 
connection  with  them.    In  the  case  of  sensor  or  afferent  nerves  the  stimuli 


48  HUMAN   PHYSIOLOGY 

act  upon  the  peculiar  end  organs  with  which  the  sensor  nerves  are  in  con- 
nection, which  in  turn  excite  the  nerve-fibers.  Experimentally,  it  can 
be  demonstrated  that  nerves  can  be  excited  by  a  sufficiently  powerful 
stimulus  applied  in  any  part  of  their  extent- 
Nerves  respond  to  stimulation  according  to  their  habitual  function; 
thus,  stimulation  of  a  sensor  nerve,  if  sufficiently  strong,  results  in  the  sen- 
sation of  pain;  of  the  optic  nerve,  in  the  sensation  of  light;  of  a  motor  nerve, 
in  contraction  of  the  muscle  to  which  it  is  distributed;  of  a  secretor  nerve, 
in  the  activity  of  the  related  gland,  etc.  It  isj  therefore,  evident  that 
peculiarity  of  nerve  function  depends  neither  upon  any  special  construc- 
tion or  activity  of  the  nerve  itself,  nor  upon  the  nature  of  the  stimulus, 
but  entirely  upon  the  peculiarities  of  its  central  and  p>eripheral  end  organs. 

Nerve  stimuli  may  be  divided  into — 

1.  General  stimtdi,  comprising  those  agents  which  are  capable  of  exciting 
a  nerve  in  any  part  of  its  course. 

2.  Special  stimuli j  comprising  those  agents  which  act  upon  nerves  only 
through  the  intermediation  of  the  end  organs. 

General  stimuli: 

1.  Mechanical:  as  from  a  blow,  pressure,  tension,  puncture,  etc. 

2.  Thermal;  heating  a  nerve  at  first  increases  and  then  decreases  its 
excitability. 

3.  Chemic:  sensor  nerves  respond  somewhat  less  promptly  than  motor 
nerves  to  this  form  of  irritation. 

4.  Electric:  either  the  constant  or  interrupted  current. 

5.  The  normal  physiologic  stimulus: 

{a)  Centrifugal  or  efferent,  if  proceeding  from  the  center  toward  the 
periphery. 

{h)  Centripetal  or  afferent,  if  in  the  reverse  direction. 

Special  stimuli: 

1.  Light  or  ethereal  vibrations  acting  upon  the  end  organs  of  the  optic 
nerve  in  the  retina. 

2.  Sound  or  atmospheric  undulations  acting  upon  the  end  organs  of 
the  auditory  nerve. 

3.  Heat  or  vibrations  of  the  air  upon  the  end  organs  in  the  skin. 

4.  Chemic  agencies  acting  upon  the  end  organs  of  the  olfactory  and 
gustatory  nerves. 


PHYSIOLOGY  OF  NERVE   TISSUE  49 

Nature  of  the  Nerve  Impulse. — As  to  the  nature  of  the  nerve  impulse 
generated  by  any  of  the  foregoing  stimuli  either  general  or  special,  but 
little  is  known.  It  has  been  supposed  to  partake  of  the  nature  of  a  mole- 
cular disturbance,  a  combination  of  physical  and  chemical  processes 
attended  by  the  liberation  of  energy,  which  propagates  itself  from  mole- 
cule to  molecule.  Judging  from  the  deflections  of  the  galvanometer  needle 
it  is  probable  that  when  the  nerve  impulse  makes  its  appearance  at  any 
given  point  it  is  at  first  feeble  but  soon  reaches  a  maximum  development 
after  which  it  speedily  declines  and  disappears.  It  may,  therefore,  be 
graphically  represented  as  a  wave-like  movement  with  a  definite  length 
and  time  duration.  Under  strictly  physiological  conditions  the  nerve 
impulse  passes  in  one  direction  only;  in  efferent  nerves  from  the  center  to 
the  periphery,  in  afferent  nerves  from  the  periphery  to  the  center.  Experi- 
mentally, however,  it  can  be  demonstrated  that  when  a  nerve  impulse  is 
aroused  in  the  course  of  a  nerve  by  an  adequate  stimulus  it  travels  equally 
well  in  both  directions  from  the  point  of  stimulation.  When  once  started 
the  impulse  is  confined  to  the  single  fiber  and  does  not  diffuse  itself  to 
fibers  adjacent  to  it  in  the  same  nerve  trunk. 

Rapidity  of  Transmission  of  Nerve  Force. — The  passage  of  a  nervous 
impulse,  either  from  the  brain  to  the  periphery  or  in  the  reverse  direction, 
requires  an  appreciable  period  of  time.  The  velocity  with  which  the 
impulse  travels  in  human  sensor  nerves  has  been  estimated  at  about  iqo 
feet  a  second,  and  for  motor  nerves  at  from  loo  to  200  feet  a  second.  The 
rate  of  movement  is,  however,  somewhat  modified  by  temperature,  cold 
lessening  and  heat  increasing  the  rapidity;  it  is  also  modified  by  electric 
conditions,  by  the  action  of  drugs,  the  strength  of  the  stimulus,  etc.  The 
rate  of  transmission  through  the  spinal  cord  is  considerably  slower  than  in 
nerves,  the  average  velocity  for  voluntary  motor  impulses  being  only  33 
feet  a  second,  for  sensitive  impressions  40  feet,  and  for  tractile  impressions 
140  feet  a  second. 

Electric  Currents  in  Muscles  and  Nerves. — If  a  muscle  or  nerve  be 
divided  and  non-polarizable  electrodes  be  placed  upon  the  natural  longi- 
tudinal surface  at  the  equator,  and  upon  the  transverse  section,  electric 
currents  are  observed  with  the  aid  of  a  delicate  galvanometer.  The  direc- 
tion of  the  current  is  always  from  the  positive  equatorial  surface  to  the 
negative  transverse  surface.  The  strength  of  the  current  increases  or 
diminishes  according  as  the  positive  electrode  is  moved  toward  or  from 
the  equator.  When  the  electrodes  are  placed  on  the  two  transverse  ends 
of  a  nerve,  an  axial  current  will  be  observed  the  direction  of  which  is  op- 
posite to  that  of  the  normal  impulse  of  the  nerve. 
4 


so  HUMAN   PHYSIOLOGY 

The  electromotive  force  of  the  strongest  nerve-current  has  been  estimated 
to  be  equal  to  the  0.026  of  a  Daniell  battery;  the  force  of  the  current  of  the 
frog  muscle,  about  0.05  to  0.08  of  a  Daniell. 

Negative  Variation  of  Currents  in  Muscles  and  Nerves. — If  a  muscle 
or  nerve  be  thrown  into  a  condition  of  tetanus,  it  will  be  observed  that  the 
currents  undergo  a  diminution  of  negative  variation,  a  change  which  passes 
along  the  nerve  in  the  form  of  a  wave  and  with  a  velocity  equal  to  the  rate 
of  transmission  of  the  nerve  impulse.  The  wave-length  of  a  single  nega- 
tive variation  has  been  estimated  to  be  eighteen  millimeters,  the  period 
of  its  duration  being  from  0.0005  to  0.0008  of  a  second. 

It  is  asserted  by  Hermann  that  perfectly  fresh,  uninjured  muscles  and 
nerves  are  devoid  of  currents,  and  that  the  currents  observed  are  the  result 
of  molecular  death  at  the  point  of  section,  this  point  becoming  negative  to 
the  equatorial  point.  He  applies  the  term  "action  currents"  to  the 
currents  obtained  when  a  muscle  is  thrown  into  a  state  of  activity. 

Electrotonus . — The  passage  of  a  direct  galvanic  current  through  a  portion 
of  a  nerve  excites  in  the  parts  beyond  the  electrodes  a  condition  of  electric 
tension,  or  electrotonus,  during  which  the  excitability  of  the  nerve  is  de- 
creased near  the  anode  or  positive  pole,  and  increased  near  the  cathode  or 
negative  pole;  the  increase  of  excitability  in  the  catelectrotonic  area — that 
nearest  the  muscle — being  manifested  by  a  more  marked  contraction  of 
the  muscle  than  the  normal  when  the  nerve  is  irritated  in  this  region.  The 
passage  of  an  inverse  galvanic  current  excites  the  same  condition  of  elec- 
trotonus; the  diminution  of  excitability  near  the  anode,  the  anelectrotonic 
— that  now  nearest  the  muscle — being  manifested  by  a  less  marked  con- 
traction than  the  normal  when  the  nerve  is  stimulated  in  this  region. 
Similar  conditions  exist  within  the  electrodes.  Between  the  electrodes  is 
a  neutral  point,  where  the  catelectrotonic  area  merges  into  the  anelectro- 
tonic area.  If  the  current  be  a  strong  one,  the  neutral  point  approaches 
the  cathode;  if  weak,  it  approaches  the  anode. 

When  a  nerve  impulse  passes  along  a  nerve,  the  only  appreciable  effect 
is  a  change  in  its  electric  condition,  there  being  no  change  in  its  tempera- 
ture, chemic  composition,  or  physical  condition.  The  natural  nerve- 
currents,  which  are  always  present  in  a  living  nerve  as  a  result  of  its 
nutritive  activity,  in  great  part  disappear  during  the  passage  of  an  impulse, 
undergoing  a  negative  variation. 

Law  of  Contraction. — If  a  feeble  galvanic  current  be  applied  to  a  recent 
and  excitable  nerve,  contraction  is  produced  in  the  muscles  only  upon  the 
making  of  the  circuit  with  both  the  direct  and  inverse  currents. 

If  the  current  be  moderate  in  intensity,  the  contraction  is  produced  in 


PHYSIOLOGY   or   NERVE   TISSUE  5 1 

,  the  muscle,  both  upon  the  making  and  breaking  of  the  circuit,  witk  both  the 
direct  ajid  inverse  currents. 

If  the  current  be  intense,  contraction  is  produced  only  when  the  circuit 
is  made  with  the  direct  current,  and  only  when  it  is  broken  with  the  inverse 
current. 

Reflex  Action. — Inasmuch  as  many  of  the  muscle  movements  of  the 
body,  as  well  as  the  formation  and  discharge  of  secretions  from  glands, 
variations  in  the  caliber  of  blood-vessels,  inhibition  and  acceleration  in 
the  activity  of  various  organs,  are  the  result  of  stimulations  of  the  terminal 
organs  of  afferent  nerves,  they  are  termed,  for  convenience,  reflex  actions, 
and,  as  they  take  place  independently  of  the  brain  or  of  volitional  impulses, 
they  are  also  termed  involuntary  actions.  As  many  of  the  processes  to 
be  described  in  succeeding  chapters  are  of  this  character,  requiring  for 
their  performance  the  cooperation  of  several  organs  and  tissues  associated 
through  the  intermediation  of  the  nervous  system,  it  seems  advisable  to 
consider  briefly,  in  this  connection,  the  parts  involved  in  a  reflex  action^ 
as  well  as  their  mode  of  action.  As  shown  in  figure  lo,  the  necessary 
structures  are  as  follows: 

1.  A  receptive  surface,  skin,  mucous  membrane,  sense  organ,  etc. 

2.  An  afferent  nerve. 

3.  An  emissive  cell,  from  which  arises 

4.  An  efferent  nerve,  distributed  to  a  responsive  organ,  as, 

5.  Muscle,  gland,  blood-vessel,  etc. 

Such  a  combination  of  structures  constitutes  a  reflex  mechanism  of 
arc  the  nerve  portion  of  which  is  composed  of  but  two  neurons — an 
afferent  and  an  efferent.  An  arc  of  this  simplicity  would  of  necessity 
subserve  but  a  simple  movement.  The  majority  of  reflex  activities,  how- 
ever, are  extremely  complex,  and  involve  the  cooperation  and  coordination 
of  a  number  of  structures  frequently  situated  at  distances  more  or  less 
remote  from  one  another.  This  implies  that  a  number  of  neurons  are 
associated  in  function.  The  afferent  neurons  are  brought  into  relation 
with  the  dendrites  of  the  efferent  neurons  by  the  end  tufts  of  the  collateral 
branches,  which  may  extend  for  some  distance  up  and  down  the  cord 
before  passing  into  the  various  segments. 

For  the  excitation  of  a  reflex  action  it  is  essential  that  the  stimulus  ap- 
plied to  the  sentient  surface  be  of  an  intensity  sufficient  to  develop  in  the 
terminals  of  the  afferent  nerve  a  series  of  nerve  impulses,  which,  traveling 
inward,  will  be  distributed  to  and  received  by  the  dendrites  of  the  emissive 
or  motor  cell.  With  the  reception  of  these  impulses  there  is  apparently  a 
disturbance  of  the  equilibrium  of  its  molecules,  a  liberation  of  energy,  and 


52  HUMAN   PHYSIOLOGY 

in  consequence,  a  transmission  outward  of  impulses  through  the  efferent 
nerve  to  muscle,  gland,  or  blood-vessel,  separately  or  collectively,  with  the 
production  of  muscular  contraction,  glandular  secretion,  vascular  dilata- 
tion or  contraction,  etc.  The  reflex  actions  take  place,  for  the  most  part, 
through  the  spinal  cord  and  medulla  oblongata,  which,  in  virtue  of  their 
contained  centers,  coordinate  the  various  organs  and  tissues  concerned  in 
the  performance  of  the  organic  functions.  The  movements  of  mastication; 
the  secretion  of  saliva;  the  muscular,  glandular,  and  vascular  phenomena 
of  gastric  and  intestinal  digestion;  the  vascular  and  respiratory  move- 
ments; the  mechanism  of  micturition,  etc.,  are  illustrations  of  reflex 
activity. 

FOODS  AND  DIETETICS 

During  the  functional  activity  of  every  organ  and  tissue  of  the  body  the 
living  material  of  which  it  is  composed — the  protoplasm — undergoes  more 
or  less  disintegration.  Through  a  series  of  descending  chemic  stages  it  is 
reduced  to  a  number  of  simpler  compounds,  which  are  of  no  further  value 
to  the  body,  and  which  are  in  consequence  eliminated  by  the  various 
eliminating  or  excretory  organs — the  lungs,  kidneys,  skin,  liver.  Among 
these  compounds  the  more  important  are  carbon  dioxid,  urea,  and  uric 
acid.  Many  other  compounds,  inorganic  as  well  as  organic,  are  also 
eliminated  in  the  water  discharged  from  the  body,  in  which  they  are  held 
in  solution.  Coincident  with  this  disintegration  of  the  tissue  there  is  an 
evolution  or  disengagement  of  energy,  particularly  in  the  form  of  heat. 

In  order  that  the  tissues  may  regain  their  normal  composition  and  thus 
be  enabled  to  continue  in  the  performance  of  their  functions,  they  must  be 
supplied  with  the  same  nutritive  materials  of  which  their  protoplasm 
originally  consisted — viz.,  water,  inorganic  salts,  proteins,  sugar,  fat. 
These  materials  are  furnished  by  the  blood  during  its  passage  through 
the  capillary  blood-vessels.  The  blood  is  a  reservoir  of  nutritive  material 
in  a  condition  to  be  absorbed,  organized,  and  transformed  into  new  living 
tissue. 

Inasmuch  as  the  loss  of  material  from  the  body  daily,  which  is  very 
great,  is  compensated  for  under  other  forms  by  the  blood,  it  is  evident  that 
this  fluid  would  rapidly  diminish  in  volume  were  it  not  restored  by  the 
introduction  of  new  and  corresponding  materials.  As  soon  as  the  blood 
volume  falls  to  a  certain  point,  the  sensations  of  hunger  and  thirst  arise, 
which  in  a  short  time  lead  to  the  necessity  of  taking  food. 

In  addition  to  the  direct  appropriation  of  food  by  the  tissues  it  is  highly 
probable  that  an  indefinite  amount  undergoes  oxidation  and  disintegra- 


FOODS   AND   DIETETICS 


53 


tion  without  ever  becoming  an  integral  part  of  the  tissues,  and  thus  directly 
contributes  to  the  production  of  heat. 

Quantities  of  Food  Materials  Required  Each  Day. — The  quantities  of 
the  different  nutritive  materials  that  are  required  each  day  for  the  growth 
and  repair  of  the  tissues  and  for  the  evolution  of  heat  have  been  variously 
estimated  by  different  observers.  The  following  table  shows  the  average 
diet  scale  of  Vierordt  and  the  amounts  of  the  waste  products  to  which  it 
would  give  rise: 

Comparison  of  the  Income  and  Outcome 


Income 


Protein 

Fat 

Starch 

Inorganic  salts 


120  grams. 

90  grams. 
330  grams. 

32  grams. 


Water 2,818  grams. 

Oxygen 744  grams. 


Outcome 

Water .  3,114.00 

Urea  33  •  80 

'  Salts  26 .  00 

Extractives  6.00 

Feces 44 .  00 

Carbon  dioxid 9 1 0 .  00 


Urinary  solids. 


4,134  00 


Total    4.134  grams. 

It  will  be  observed  that  in  the  results  of  the  foregoing  experiment, 
the  amount  of  water  under  outcome,  exceeds  the  amount  under  income, 
by  296  grams.  This  water  results  from  the  union  of  a  portion  of  the 
oxygen  absorbed  with  the  surplus  hydrogen  of  the  fats.  If  the  diet 
consisted  merely  of  protein  and  starch  the  two  volumes  of  water  would 
practically  balance  each  other. 

Many  other  attempts  have  been  made  to  construct  a  suitable  diet  for 
a  man  weighing  70  kilos  while  doing  light  or  moderate  work.  The 
following  are  accepted  estimates: 


Ranke, 
grams 


Voit, 

grams 


Moleschott, 
grams 


Atwater, 
grams 


Protein 

Fat 

Starch 


100 
100 

250 


118 

S6 

500 


130 

84 

550 


125 
125 
400 


The  Energy  of  the  Animal  Body. — The  food  consumed  daily  not  only 
repairs  the  loss  of  material  from  the  body,  but  also  furnishes  the  energy  to 
replace  that  which  is  expended  daily  in  the  shape  of  heat  of  motion. 
All  the  energy  of  the  body  can  be  traced  to  the  chemic  changes  going  on 
in  the  tissues,  and  more  particularly  to  those  changes  involved  in  the 
oxidation  of  the  foods. 


54  HUMAN  PHYSIOLOGY 

The  amount  of  heat  yielded  by  any  given  food  principle  can  be  deter- 
mined by  burning  it  to  carbon  dioxid  and  water,  and  ascertaining  the 
extent  to  which  it  will,  when  so  liberated,  raise  the  temperature  of  a  given 
volume  of  water.  This  amount  of  heat  .may  be  expressed  in  Calories. 
A  Calorie  is  the  amount  of  heat  required  to  raise  the  temperature  of  one 
kilogram  of  water  one  degree  Centigrade. 

The  following  estimates  give,  approximately,  the  number  of  Calories 
produced  when  the  food  is  reduced  within  the  body  to  urea,  carbon  dioxid, 
and  water: 


I  gram  of  protein  yields 4  •  124  kilogram  Calories. 

I  gram  of  fat  yields 9-353  kilogram  Calories. 

I  gram  of  starch  yields 4 . 1 16  kilogram  Calories. 


The  total  number  of  kilogram  Calories  yielded  by  any  given  diet  scale 
can  be  readily  determined  by  multiplying  the  preceding  factors  by  the 
quantities  of  material  consumed.  The  diet  scale  of  Ranke,  for  example, 
yields  the  following  amount: 


ioo  grams  of  protein  yield    412 .4  Calories. 

100  grams  of  fat  yield 935 .3  Calories. 

240  grams  of  starch  yield    987 . 8  Calories. 

Total 2,335  S  Calories. 


It  has  also  been  determined  experimentally  that  one  gram  of  protein, 
one  gram  of  fat,  and  one  gram  of  starch,  when  completely  oxidized,  will 
yield  energy  sufl&cient  to  perform,  1,850,  3,841  and  1,567  kilogrammeters 
of  work,  respectively.  A  kilogramme ter  of  work  is  one  kilogram  raised 
one  meter  high. 

The  total  energy  of  the  Ranke  diet  scale  can  be  easily  calculated — e.g.y 

100  grams  of  protein  yield 185,000  kilogrammeters. 

100  grams  of  fat  yield 384,100  kilogrammeters. 

240  grams  of  starch  yield 397t68o  kilogrammeters. 

Total 966,780  kilogrammeters. 

It  will  be  thus  seen  that  the  food  consumed  daily  yields  2,335  kilogram 
CalorieSf  which  can  be  translated  into  its  mechanical  equivalent  966,780 
kilogrammeters  of  work. 


FOODS   AND   DIETETICS  55 

CLASSIFICATON  OF  FOOD  PRINCIPLES 

1.  Proteins. 

Principle  \»'iiere  found 

Myosin Flesh  of  animals. 

Vitellin,  albumin Yolk  of  egg,  white  of  egg. 

Fibrin,  globulin Blood  contained  in  meat. 

Caseinogen Milk,  cheese. 

Gliadin  and  glutinin Grain  of  wheat  and  other  cereals. 

Vegetable  albumin Soft,  growing  vegetables. 

Legumin Peas,  beans,  lentils,  etc. 

2.  Fats. 

Animal  fats  and  oils 1  Found  in  the  adipose  tissue  of  animals, 

Stearin,  olein |-      seeds,    grains,    nuts,    fruits    and    other 

Palmitin,  fat  acids      J      vegetable  tissues. 

3.  Carbohydrates. 

Saccharose,  or  cane-sugar Sugar-cane. 

Dextrose,  or  glucose 1  _     . 

Levulose,  or  fruit-sugar / 

Lactose,  or  milk-sugar    Milk. 

Maltose Malt,  malt  foods. 

Starch Cereals,   tuberous   roots,   and   leguminous 

plants 
Glycogen Liver,  muscles. 

4.  Inorganic  Principles. — Water;  sodium  and  potassium  chlorids; 
sodium  calcium,  magnesium,  and  potassium  phosphates;  calcium  carbon- 
ate; and  iron. 

5.  Vegetable  Acids. — Malic,  citric,  tartaric,  and  other  acids,  found 
principally  in  fruits. 

6.  Accessory  Foods. — Tea,  coffee,  alcohol,  cocoa,  etc. 

DISPOSITION  OF  FOOD 

The  Proteins. — The  protein  principles  of  the  food  while  in  the  ali- 
mentary canal  undergo  a  series  of  disintegrative  changes  by  virtue  of  which 
they  are  reduced  in  part  to  simple  nitrogen-holding  bodies,  monoamino- 
and  diamino-acids  and  ammonia,  and  in  part  to  their  immediate  anteced- 
ents peptids  and  polypeptids,  after  which  they  are  absorbed  from  the 
intestinal  contents.  Recently  evidence  has  been  adduced  which  makes  it 
probable  that  the  amino-acids  undergo  no  change  in  the  act  of  absorption 
but  enter  the  blood  as  such  and  are  carried  direct  to  the  tissues.  On 
reaching  any  given  tissue  the  cells  absorb  and  synthesize,  perhaps  under 


56  HUMAN  PHYSIOLOGY 

the  influence  of  an  enzyme,  such  amino-acids  as  they  may  need  for  their 
growth  and  repair.  The  surplus  amino-acids,  i.e.,  those  not  utilized  in 
the  synthesis  of  tissue  protein,  may  be  synthesized  to  plasma-albumin, 
or  stored  unchanged  or  be  deaminized,  i.e.,  separated  perhaps  by  the 
action  of  an  enzyme,  into  the  amino-group,  NH2,  and  some  carbonaceous 
radical.  The  amino-group  is  then  combined  with  hydrogen,  and  sub- 
sequently with  carbon  dioxid,  to  form  ammonium  carbonate  which 
is  then  transformed  into  urea,  a  transformation  that  takes  place  to  some 
extent  in  the  muscles  (Folin) ;  the  carbonaceous  remainder  may  be  trans- 
formed into  fat  or  sugar,  which  is  subsequently  oxidized  thus  contributing 
to  the  production  of  heat.  In  the  process  of  tissue  metabolism  the  protein 
molecule  undergoes  disintegration  and  gives  rise  to  amino-acids,  the 
different  elements  of  which  may  undergo  changes  similar  to  those  just 
stated.  The  ammonia  absorbed  from  the  intestine  is  changed  to  ammon- 
ium carbonate  carried  direct  to  the  liver  and  transformed  into  urea. 

The  Fats. — The  fat  principles  while  in  the  alimentary  canal  also  undergo 
a  series  of  changes  whereby  they  are  reduced  by  enzymic  action  to  soap 
and  glycerin,  under  which  forms  they  are  absorbed.  During  the  act  of 
absorption  the  soap  and  glycerin  are  synthesized  to  human  fat.  The  fine 
particles  thus  formed  in  the  intestinal  wall  are  carried  by  the  lymph  vessels 
to  the  thoracic  duct,  and  thence  into  the  blood  stream,  from  which  they 
rapidly  disappear.  Though  it  is  possible  that  a  portion  of  the  fat  enters 
directly  into  the  formation  of  the  living  material  in  general,  it  is  generally 
believed  that  it  is  at  once  oxidized  and  reduced  to  carbon  dioxid  and  water 
with  the  liberation  of  energy.  The  natural  supposition  that  a  portion  of 
the  synthesized  fat  is  directly  stored  up  in  the  cells  of  the  areolar  connect- 
ive tissue,  thus  giving  rise  to  adipose  tissue,  has  been  a  subject  of  much 
controversy,  though  modern  experimentation  renders  this  very  probable. 
The  body-fat,  under  physiologic  conditions,  is  mainly,  however,  a  product 
of  the  transformation  of  carbohydrates. 

The  Carbohydrates. — Carbohydrate  principles  are  reduced  during  di- 
gestion to  simple  forms  of  sugar,  chiefly  dextrose  and  levulose.  Under 
these  forms  they  are  absorbed  into  the  blood.  These  compounds  are  then 
carried  to  the  liver  and  to  the  muscles  where  they  are  dehydrated  and 
stored  under  the  form  of  starch,  termed  animal  starch  or  glycogen.  Sub- 
sequently glycogen  is  transformed  by  hydration  to  sugar,  after  which  it 
is  oxidized  to  carbon  dioxid  and  water.  The  intermediate  stages  through 
which  sugar  passes  before  it  is  reduced  to  carbon  dioxid  and  water  are 
only  imperfectly  known.  Though  a  large  part  of  the  carbohydrate 
material  is  at  once  oxidized,  it  is  now  well  established  that  another  por- 


FOODS    AND   DIETETICS  57 

tion  contributes  to  the  formation  of,  if  it  is  not  directly  converted  into,  fat. 
As  the  carbohydrates  form  a  large  portion  of  the  food,  they  contribute 
materially  to  the  liberation  of  energy. 

The  Inorganic  Principles. — The  inorganic  principles,  though  apparently 
not  playing  as  active  a  part  in  the  metabolism  of  the  body  as  the  organic, 
are  nevertheless  essential  to  its  physiologic  activity. 

Water  is  present  in  all  the  fluids  and  solids  of  the  body.  It  promotes  the 
absorption  of  new  material  from  the  alimentary  canal;  it  holds  the  various 
ingredients  of  the  blood,  lymph,  and  other  fluids  in  solution;  it  hastens  the 
absorption  of  waste  products  from  the  tissues,  and  promotes  their  speedy 
elimination  from  the  body. 

Sodium  chlorid  is  present  in  all  parts  of  the  body  to  the  extent  of  no 
gm.  The  average  amount  eliminated  daily  is  15  gm.  Its  necessity  as  an 
article  of  diet  is  at  once  apparent.  Taken  as  a  condiment,  it  imparts 
sapidity  to  the  food,  excites  the  flow  of  the  digestive  fluids,  influences 
the  passage  of  nutritive  material  through  animal  membranes,  and  fur- 
nishes the  chlorin  for  the  free  hydrochloric  acid  of  the  gastric  juice.  In 
some  unknown  way  it  favorably  promotes  the  activity  of  the  general 
nutritive  process. 

The  potassium  salts  are  also  essential  to  the  normal  activity  of  the 
nutritive  process.  When  deprived  of  these  salts,  animals  beome  weak 
and  emaciated.  When  given  in  small  doses,  they  increase  the  force  of 
the  heart-beat,  raise  the  arterial  pressure,  and  thus  increase  the  action  of 
the  circulation  of  the  blood. 

The  calcium  phosphate  and  carbonate  are  utilized  in  imparting  solidity  to 
the  tissues,  more  especially  the  bones  and  teeth.  Many  articles  of  food 
contain  these  salts  in  quantities  sufficient  to  restore  the  amount  lost 
daily. 

The  vegetable  acids  increase  the  secretions  of  the  alimentary  canal,  and 
are  apt,  in  large  amounts,  to  produce  flatulence  and  diarrhea.  After 
entering  into  combination  with  bases  to  form  salts,  they  stimulate  the 
action  of  the  kidneys  and  promote  a  greater  elimination  of  all  the  urinary 
constituents.  In  some  unknown  way  they  influence  nutrition;  when 
deprived  of  these  acids,  the  individual  becomes  scorbutic. 

The  accessory  foods,  coffee  and  tea,  when  taken  in  moderation,  overcome 
the  sense  of  fatigue  and  mental  unrest  consequent  on  excessive  physical 
and  mental  exertion.  Coffee  increases  the  action  of  the  intestinal  glands 
and  acts  as  a  laxative.  After  absorption,  its  active  principle,  cafein 
stimulates  the  action  of  the  heart,  raises  the  arterial  pressure,  and  excites 
the  action  of  the  brain.    Tea  acts  as  an  astringent,  owing  to  the  tannic 


58  HUMAN   PHYSIOLOGY 

acid  it  contains.  One  effect  of  the  tannic  acid  is  to  coagulate  the  digestive 
ferments  and  to  interfere  with  the  activity  of  the  digestive  process. 

Alcohol  when  taken  in  small  quantities  stimulates  the  digestive  glands 
to  increased  activity  and  thus  promotes  digestive  power.  Its  absorption 
into  the  blood  is  followed  by  increased  action  of  the  heart,  dilatation  of 
the  cutaneous  blood-vessels,  a  sensation  of  warmth,  and  an  excitation  of 
the  brain.  In  large  quantities  it  acts  as  a  paralyzant,  depressing  more 
especially  the  vaso-constrictor  nerve-centers  and  certain  areas  of  the  brain, 
as  shown  by  an  impairment  in  the  power  of  sustained  attention,  clearness 
of  judgment,  and  muscle  coordination. 

Alcohol  is  undoubtedly  oxidized  in  the  body,  as  only  about  2  per  cent, 
can  be  obtained  from  the  urine  and  expired  air.  It  thus  contributes  to  the 
store  of  the  body-energy.  Whether  for  this  reason  it  can  be  regarded 
as  a  food — that  is,  whether  it  can  be  substituted  in  part  at  least  for  fat  or 
carbohydrate  material  without  impairing  the  protein  metabolism — is  at 
present  a  subject  of  experimentation  and  discussion.  According  to  some 
investigators,  alcohol  does  not  retard  protein  metabolism,  for  when  it  is 
introduced  into  the  body  in  amounts  equivalent  to  the  carbohydrates  with- 
drawn from  the  food  there  is  at  once  a  rise  in  the  amount  of  nitrogen 
excreted.  Hence  it  cannot  be  regarded  as  a  food.  According  to  other 
investigators,  alcohol  retards  or  protects  protein  metabolism  just  as 
effectually  as  an  equivalent  amount  of  starch  or  sugar.  Many  more 
experiments  are  required  to  decide  this  question.  When  taken  habitually 
in  large  quantities,  alcohol  deranges  the  activities  of  the  digestive  organs, 
lowers  the  body  temperature,  impairs  muscle  power,  lessens  the  resistance 
to  depressing  external  conditions,  diminishes  the  capacity  for  sustained 
mental  work,  and  leads  to  the  development  of  structural  changes  in  the 
connective  tissues  of  the  brain,  spinal  cord,  and  other  organs.  In  infec- 
tious diseases  and  in  cases  of  depression  of  the  vital  powers  it  is  most  use 
ful  as  a  restorative  agent. 

Inanition  or  Starvation. — If  these  nutritive  principles  be  not  supplied 
in  sufficient  quantity,  or  if  they  are  withheld  entirely,  a  condition  of  physio- 
logic decay  is  established,  to  which  the  term  inanition  or  starvation  is 
applied.  The  phenomena  which  characterize  this  pathologic  process  are 
as  follows — viz.,  hunger,  intense  thirst,  gastric  and  intestinal  uneasiness 
and  pain,  muscle  weakness  and  emaciation,  a  diminution  in  the  quantity 
of  carbon  dioxid  exhaled,  a  lessening  in  the  amount  of  urine  and  its  con- 
stituents excreted,  a  diminution  in  the  volume  of  the  blood,  an  exhalation 
of  a  fetid  odor  from  the  body,  vertigo,  stupor,  delirium,  and  at  times  con- 
vulsions, a  fall  of  bodily  temperature,  and,  finally,  death  from  exhaustion. 


FOODS   AND   DIETETICS  59 

During  starvation  the  loss  of  different  tissues,  before  death  occurs, 
averages  ^o,  or  40  per  cent.,  of  their  weight. 

Those  tissues  which  lose  more  than  40  per  cent,  are:  Fat,  93.3;  blood,  75; 
spleen,  71.4;  pancreas,  64.1;  liver,  52;  heart,  44.8;  intestines,  42.4;  muscle, 
42.3.  Those  which  lose  less  than  40  per  cent,  are:  The  muscular  coat  of 
the  stomach,  39.7;  pharynx  and  esophagus,  34.2;  skin,  33.3;  kidneys, 
31.9;  respiratory  apparatus,  22.2;  bones,  16.7;  eyes,  10;  nervous  system, 
1.9 

The  fat  entirely  disappears,  with  the  exception  of  a  small  quantity 
which  remains  in  the  posterior  portion  of  the  orbits  and  around  the  kidneys. 
The  blood  diminishes  in  volume  and  loses  its  nutritive  properties.  The 
muscles  undergo  a  marked  diminution  in  volume  and  become  soit  and 
flabby.  The  nervous  system  is  last  to  suffer,  not  more  than  two  per  cent., 
disappearing  before  death  occurs. 

The  appearances  presented  by  the  body  after  death  from  starvation  are 
those  of  anemia  and  great  emaciation;  almost  total  absence  of  fat;  blood- 
lessness;  a  diminution  in  the  volume  of  the  organs;  an  empty  condition  of 
the  stomach  and  bowels,  the  coats  of  which  are  thin  and  transparent. 
There  is  a  marked  disposition  of  the  body  to  undergo  decomposition,  giving 
rise  to  a  very  fetid  odor. 

The  duration  of  life  after  a  complete  deprivation  of  food  varies  from  eight 
to  thirteen  days,  though  life  can  be  maintained  much  longer  if  a  quantity 
of  water  be  obtained.  The  water  is  more  essential  under  these  circum- 
stances than  the  solid  matters,  which  can  be  supplied  by  the  organism 
itself. 


COMPOSITION  OF  FOODS 

The  food  principles  essential  to  the  maintenance  of  the  nutrition  of 
the  body  are  contained  in  varying  proportions  in  compound  substances 
termed  foods;  e.g.,  meat,  milk,  wheat,  potatoes,  etc.  Their  nutritive  value 
depends  partly  on  the  amounts  of  their  contained  food  principles  and 
partly  on  their  digestibility.  The  dietary  of  civilized  man  embraces  foods 
derived  from  both  the  animal  and  vegetable  worlds. 

The  following  tables  show  the  percentage  composition  of  the  edible 
portions  of  foods  as  well  as  the  amount  of  heat  liberated  per  pound  when 
oxidized  in  the  body,  according  to  Atwater  and  Bryant. 

Composition  of  Animal  Foods. — The  following  table  shows  the  average 
percentage  composition  of  various  kinds  of  meats,  cow's  milk,  and  eggs: 


6o 


HUMAN   PHYSIOLOGY 


Kind  of  food 
materials 


Water 


Unavail- 
able 
nutrients 


Pro- 
teins 


Fat 


Car- 
bohy- 
drates 


Ash 


Fuel 
value  per 
lb.,  453.6 

grams 


Beef: 

Loin,  lean 

Loin,  fat 

Round,  lean 

Round,  fat 

Veal: 

Cutlets  (round) 

Liver 

Mutton : 

Leg 

Loin    

Pork: 

Loin  chops  .... 

Ham 

Fowl: 

Turkey 

Mackerel 

Halibut    

Milk 

Eggs,  boiled 


Per 
cent. 

67.0 
54-7 
70.0 
60.4 

70.7 
73.0 

62.8 
50.2 

52. o 
S3. 9 
63.7 
5SS 

73.4 
75.4 
87.0 
73.2 


Per 
cent. 


Per 

cent. 


I 


1.2  I  19. 1 

1.9  i  17.0 

i.o  j  20.7 

1.6  j  18.9 


I  -3 
0.9 


I 


19.7 
9.7 

17.9 
IS. 5 

16. 1 
14.8 
18.7 

20.5 

18. 1 

18.0 

3.2 

12.8 


Per 
cent. 

12. 1 

26.2 

75 


7.3 
5.0 

17. 1 
31.4 

28.6 

27. S 

155 
21.8 
6.7 
4-9 
3.8 
II. 4 


Per 

cent. 


Per 

cent, 


0.8 
0.6 


Calories 


900 

1,470 

735 

1. 175 

710 
410 

1,095 
1,660 

1,555 
1,480 
1,040 
853 
650 
570 
310 
755 


Composition  of  Cereal  Foods. — The  average  composition  of  the  prin- 
cipal cereals  is  shown  in  the  following  table : 


Kind  of  food 
material 


Water 


Unavail- 
able 
nutrients 


Pro- 
teins 


Fat 


Car- 
bohy- 
drates 


Fuel 
A  „i,   i     value  per 
A«^  I     lb..  453.6 
I         grams 


Entire  wheat  flour. 

Rye  flour 

Rice    

Barley,  pearled.  . .  . 
Buckwheat  flour. . . 

Corn  meal 

Oat  meal    

Whole  wheat  bread 

White  bread    

Graham  crackers    . 


Per 
cent. 

11. 4 
12.9 
12.3 

11. 5 
13.6 
12. S 

7.8 
38.4 
35. 3 

5-4 


Per 
cent. 
4.5 
3 
3 
4 
3 
4 
5 
3 
3 
4 


Per 

Per 

Per 

cent. 

cent. 

cent. 

10.7 

1-7 

70.9 

5.3 

0.8 

76.9 

6.5 

0.3 

76.9 

6.6 

1.0 

76.10 

5.2 

I .  I 

75.9 

7.5 

1.7 

73. 5 

13.4 

6.6 

65.2 

7.5 

0.8 

49.1 

7.1 

1.2 

52.3 

7.7 

8.5 

72.5 

Per 

cent. 
0.8 


0.8 


Calories 

1,64s 
1,610 
1,610 
1,630 
1,600 
1.62s 
1. 795 
1. 125 
1. 195 
1,900 


DIGESTION 


6l 


Composition  of  Vegetable  Foods. — The  average  composition  of  some 
of  the  principal  vegetables  is  shown  in  the  following  table; 


Kind  of  food 
material 


Beans,  lima,  dried 
Beans,  lima,  green . 
Beans,  white, 

dried 

Beans,  string, 

cooked^  

Peas,  dried 

Peas,  green, 

cooked^  

Potatoes,  boiled, 

cooked'  

Potatoes,  sweet  .  .  . 
Beets,   cooked ^  .  .  . 

Cabbage  

Tomatoes 

Turnips    

Egg-plant 

Spinach,  fresh 

Asparagus,  cooked. 


I    Unavail- 
Water        able 

nutrients 


Per 
cent. 
10.4 
68.5 

12.6 

95.3 
9  5 

73.8 

75. 5 
51.9 
88.6 
91.5 
94-3 
89.6 
92.9 
92.3 
91.6 


Per 

cent. 

6.7 

2.7 

7.5 

0.5 
7.6 


Pro- 
teins 


Fat 


Per 
cent. 
12.8 

5.3 

15.8 

0.6 
17.3 


Per 

cent. 

1.4 

0.6 

1.6 

i.o 
0.9 


1 .9 
0.1 


.2 

0.3 

.7 

0.4 

.0 

0.2 

9 

0.3 

.6 

0.3 

.7 

30 

Car- 
bohy- 
drates 


Per 
cent. 
65.6 
21 .6 


1.9 
62.5 

14.4 

20.0 
40.3 
7.2 
5.5 
3.8 
7.8 
4.9 
3.2 
2.1 


Ash 


Per 
cent. 
3.1 
1.3 

2.6 


0.8 
0.7 
1.2 
0.8 
0.4 
0.6 
0.4 
1.6 
0.6 


Fuel 

value  per 

lb.,  453-6 

grams 


Calories 

1.56s 
52s 


90 
1,508 

490 

415 
885 
170 
140 
100 
175 
120 
100 
195 


iWith  butter,  etc.,  added. 


DIGESTION 

Digestion  is  a  physical  and  chemic  process  by  which  the  food  introduced 
into  the  alimentary  canal  is  liquefied  and  its  nutritive  principles  trans- 
formed by  the  digestive  fluids  into  new  substances  capable  of  being  ab- 
sorbed into  the  blood. 

The  digestive  apparatus  consists  of  the  alimentary  canal  and  its  appen- 
dages— viz.,  teeth,  lips  and  tongue;  the  salivary,  gastric  and  intestinal 
glands;  the  liver  and  pancreas. 

Digestion  may  be  divided  into  several  stages;  prehension,  mouth 
digestion  (mastication  and  insaliv^ation),  deglutition,  gastric  and  intestinal 
digestion,  and  defecation. 

Prehension,  the  act  of  conveying  food  into  the  mouth,  is  accomplished 
by  the  hands,  lips,  and  teeth. 


62  HUMAN  PHYSIOLOGY 

Mouth  Digestion. — Mastication  is  the  mechanical  division  of  the  food, 
and  is  accomplished  by  the  teeth  and  the  movements  of  the  lower  jaw  under 
the  influence  of  muscular  contraction.  When  throughly  divided,  the 
food  presents  a  larger  surface  for  the  solvent  action  of  the  digestive  fluids, 
thus  enabling  them  to  exert  their  respective  action  more  effectively  and 
in  a  shorter  period  of  time. 

The  teeth  are  thirty-two  in  number,  sixteen  in  each  jaw,  and  divided 
into  four  incisors  or  cutting  teeth,  two  canines,  four  bicuspids,  and  six 
molars  or  grinding  teeth;  each  tooth  consists  of  a  crown  covered  by  enamel, 
a  neck,  and  a  root  surrounded  by  the  crusta  petrosa  and  embedded  in  the 
alveolar  process;  a  section  through  a  tooth  shows  that  its  substance  is 
made  of  dentine^  in  the  center  of  which  is  the  pulp  cavity  containing  blood- 
vessels and  nerves. 

The  lower  jaw  is  capable  of  making  a  downward  and  an  upward,  a  lateral 
and  an  anteroposterior  movement,  dependent  upon  the  construction  of  the 
temporomaxillary  articulation. 

The  jaw  is  depressed  by  the  contraction  of  the  digastric,  geniohyoid, 
mylohyoid y  and  platysma  myoides  muscles;  elevated  by  the  temporal ,  mas- 
seter,  and  internal  pterygoid  muscles;  moved  laterally  by  the  alternate 
contraction  of  the  external  pterygoid  muscles;  moved  anteriorly  by  the 
pterygoid f  and  posteriorly  by  the  united  actions  of  the  geniohyoid,  mylohyoid, 
and  posterior  fibers  of  the  temporal  muscles. 

The  food  is  kept  between  the  teeth  by  the  intrinsic  and  extrinsic  muscles 
of  the  tongue  from  within,  and  the  orbicularis  oris  and  buccinator  muscles 
from  without. 

The  movements  of  mastication,  though  originating  in  an  effort  of  the 
will  and  under  its  control,  are,  for  the  most  part,  of  an  automatic  or  reflex  ^ 
character,  taking  place  through  the  medulla  oblongata  and  induced  by  the 
presence  of  food  within  the  mouth.    The  nerves  and  nerve-centers  in- 
volved in  this  mechanism  are  shown  in  the  following  table: 

Nerve  Mechanism  of  Mastication 


Afferent  nerves 

Efferent  nerves 

I.  Lingual  branches  of  the  tri- 

I. Small  root  of  the    trigeminal 

geminal  nerve. 

nerve. 

2.  Glossopharyngeal. 

2.  Hypoglossal. 

3.  Facial. 

The  impressions  made  upon  the  terminal  filaments  of  the  afferent  nerves 
are  transmitted  to  the  medulla;  motor  impulses  are  here  generated  which 
are  transmitted  through  the  efferent  nerves  to  the  muscles  involved  in  the 


DIGESTION  63 

movements  of  the  lower  jaw.  The  medulla  not  only  generates  motor 
impulses,  but  coordinates  them  in  such  a  manner  that  the  movements  of 
mastication  may  be  directed  toward  the  accomplishment  of  a  definite 
purpose. 

Insalivation. — Insalivation  is  the  incorporation  of  the  food  with  the 
saliva  secreted  by  the  paroHdy  submaxillary^  and  sublingual  glands;  the 
parotid  saliva,  thin  and  watery,  is  poured  into  the  mouth  through  Steno's 
duct;  the  submaxillary  and  sublingual  salivas,  thick  and  viscid,  are  poured 
into  the  mouth  through  Wharton's  and  Bartholin's  ducts  respectively. 

In  their  minute  structure  the  salivary  glands  resemble  one  another. 
They  belong  to  the  racemose  variety,  and  consist  of  small  sacs  or  vesicles, 
which  are  the  terminal  expansions  of  the  smallest  salivary  ducts.  Each 
vesicle  or  acinus  consists  of  a  basement  membrane  surrounded  by  blood- 
vessels and  lined  with  epithelial  cells.  In  the  parotid  gland  the  lining 
cells  are  granular  and  nucleated;  in  the  submaxillary  and  sublingual  glands 
the  cells  are  large,  clear,  and  contain  a  quantity  of  mucigen.  During  and 
after  secretion  very  remarkable  changes  take  place  in  the  cells  lining  the 
acini,  which  are  in  some  way  connected  with  the  essential  constituents  of 
the  salivary  fluids. 

In  the  living  serous  glands — e.g.,  parotid — during  rest,  the  secretory 
cells  lining  the  acini  of  the  gland  are  seen  to  be  filled  with  fine  granules, 
which  are  often  so  abundant  as  to  obscure  the  nucleus  and  enlarge  the  cells 
until  the  lumen  of  the  acinus  is  almost  obliterated.  When  the  gland 
begins  to  secrete  the  saliva,  the  granules  disappear  from  the  outer 
boundary  of  the  cells,  which  then  become  clear  and  distinct.  At  the  end  of 
the  secretory  activity  the  cells  have  been  freed  of  granules  and  have 
become  smaller  and  more  distinct  in  outline.  It  would  seem  that  the 
granular  matter  is  formed  in  the  cells  during  the  period  of  rest  and  dis- 
charged into  the  ducts  during  the  activity  of  the  gland. 

In  the  mucous  glands — e.g.,  submaxillary  and  sublingual — the  changes 
that  occur  in  the  cells  are  somewhat  different.  During  the  inter- 
vals of  digestion  the  cells  lining  the  gland  are  large,  clear,  and 
highly  refractive,  and  contain  a  large  quantity  of  mucigin.  After  secre- 
tion has  taken  place  the  cells  exhibit  a  marked  change.  The  mucigin 
cells  have  disappeared,  and  in  their  place  are  cells  which  are  small,  dark, 
and  composed  of  protoplasm.  It  would  appear  that  the  cells,  during  rest, 
elaborate  the  mucigin,  which  is  discharged  into  the  tubules  during  secretory 
activity,  to  become  part  of  the  secretion. 

Mouth  Saliva. — The  saliva  found  in  the  mouth  is  an  opalescent,  slightly 
viscid,  alkaline  fluid,  having  a  specific  gravity  of  1.005.     Microscopic 


64  HUMAN   PHYSIOLOGY 

examination  reveals  the  presence  of  salivary  corpuscles  and  epithelial 
cells.  Chemically  it  is  composed  of  water  protein  materials  and  inorganic 
salts.     The  amount  secreted  daily  has  been  estimated  at  about  2  lb. 

Physiologic  Action. — Experiments  have  shown  that  saliva  has  a  two- 
fold action,  viz.,  physical  and  chemical. 

1.  Physically  saliva  moistens  and  softens  the  food,  unites  its  particles 
into  a  consistent  mass  and  thus  facilitates  swallowing. 

2.  Chemically  it  converts  boiled  starch  into  sugar.  It  has  a  feeble  if 
any  action  on  raw  starch  by  reason  of  the  structure  of  the  starch  granule. 
Each  granule  consists  of  two  portions,  an  envelope  of  cellulose  and  a 
contained  material  granulose,  the  true  starch  material.  When  subjected 
to  the  action  of  boihng  water  the  granule  swells  and  bursts  forming  a 
more  or  less  viscid  fluid,  starch  paste.  If  saliva  be  now  added  to  this  paste 
and  kept  at  a  temperature  of  about  ioo°F.  for  a  few  minutes,  the  paste 
becomes  clear  and  liquid.  The  first  stage  in  the  digestion  of  starch  is 
now  complete  with  the  formation  of  soluble  starch.  If  the  action  of  saliva 
be  continued,  substances  intermediate  between  starch  and  sugar  are 
formed  to  which  the  name  dextrin  has  been  given,  e.g., 

{  V     fVi    A     f  •     —J  Achroodextrin 

Starch  =  Soluble  starch  =  s  ,,  ,,  1  Maltose. 

\  Maltose  ^ 

The  erythrodextrin  is  so  called  because  it  gives  rise  to  a  red  color  with 
iodin.     Achroodextrin  is  so  called  because  it  yields  no  color  with  iodin. 

The  sugar  formed  by  the  action  of  saliva  is  the  compound  sugar  maltose 
the  formula  for  which  is  C12H22O11.  This  chemical  action  of  saliva 
depends  on  the  presence  of  an  unorganized  ferment  or  enzyme  known  as 
ptyalin. 

Nerve  Mechanism  for  the  Secretion  of  Saliva. — The  afferent  and  effer- 
ent nerves  that  constitute  the  nerve  mechanism  for  the  secretion  of  saliva 
are  shown  in  the  following  tabulation : 

Afferent  nerves  Nerve  center    ^  ^       Efferent  nerves 

1.  Lingual  branches  of  the        Medulla  i.  Auriculotemporal  branch 
trigeminal  nerve.                    oblongata.  of  the  trigeminal  nerve, 

2.  Taste  fibers  in  the  glosso-  for  parotid  gland. 

' —  pharyngeal.  2.  Chorda  tympani,  for  sub - 

3.  Taste  fibers  in  the  chorda  maxillary  and  sublingual 
-  tympani.                                                               glands. 

3.  Sympathetic  for  all  the 
glands. 


DIGESTION  65 

The  nerve  centers  exciting,  through  efferent  nerves  the  secretion  of  saliva 
are  located  in  the  medulla  oblongata  and  may  be  aroused  to  action  (i)  by 


nerve  impulses  descending  from  the  brain  in  consequence  of  psychic  stoics 
induced  by  the  sight  and  odor  of  food  and  (2)  by  nerve  impulses  reflected 
through  afferent  nerves  from  the  mouth  developed  by  the  taste  of  food. 
The  afferent  nerves  thus  stiniulated  in  the  second  instance  are  those  stated 
in  the  foregoing  tabulation. 

That  the  efferent  nerves  in  the  same  tabulation  are  active  in  the  produc- 
tion of  the  secretion  is  shown  by  the  following  facts: 

Stimulation  of  the  auriculotemporal  branch  increases  the  flow  of  saliva 
from  the  parotid  gland;  division  arrests  it. 

Stimulation  of  the  chorda  tynipani  is  followed  by  a  dilatation  of  the; 
blood-vessels  of  the  submaxillary  and  sublingual  glands,  an  increased  flow 
of  blood  and  an  abundant  discharge  of  skliva;  division  of  the  nerve  arrests 
the  secretion. 

Stimulation  of  the  cervical  sympathetic  is  followed  by  a  contraction  of 
the  blood-vessels,  a  diminished  flow  of  blood,  and  a  diminution  of  the 
secretion,  which  now  becomes  thick  and  viscid;  division  of  the  sympathetic 
is  not,  however,  followed  by  complete  dilatation  of  the  vessels.  There  is 
evidence  of  the  existence  of  a  local  vasomotor  mechanism,  which  is^ 
inhibited  by  the  chorda  tympani. 

IJeglutition. — Deglutition  is  the  act  of  transferring  food  from  the  mouth 
into  the  stomach,  and  may  be  divided  into  three  stages: 

1.  The  passage  of  the  bolus  from  the  mouth  into  the  pharynx. 

2.  From  the  pharynx  into  the  esophagus. 

3.  From  the  esophagus  into  the  stomach. 

In  the  first  stage,  which  is  entirely  voluntary,  the  mouth  is  closed  and 
respiration  momentarily  suspended;  the  tongue,  placed  against  the  roof 
of  the  mouth,  arches  upward  and  backward,  and  forces  the  bolus  into 
the  fauces. 

The  second  and  third  stages,  or  the  passage  of  the  food  through  the 
pharynx  and  esophagus  into  the  stomach,  have  been  attributed  until  quite 
recently  entirely  to  peristaltic  movements  of  their  musculature. 

Recent  experiments  have  demonstrated  that  deglutition  consists  of  two 
phases:  (i)  a  rapid  rise  of  pressure  in  the  pharynx,  as  a  result  of  which 
liquid  or  semi-liquid  foods  are  suddenly  shot  down  to  the  lower  end  of  the 
esophagus;  (2)  a  peristaltic  contraction  of  the  musculature  of  the  canal, 
which,  acting  as  a  supplementary  force,  carries  onward  any  particles  of 
food  in  the  canal  and  forces  the  bolus  through  the  closed  sphincter  cardice 
at  the  end  of  the  esophagus. 
5 


66  HUMAN   PHYSIOLOGY 

The  immediate  cause  of  the  sudden  rise  of  pressure  was  shown  to  be 
the  contraction  of  the  mylohyoid  muscles.  When  the  nerves  going  to 
these  muscles  were  divided  in  a  dog,  deglutition  was  practically  abolished. 
These  muscles  are  probably  assisted  in  their  action  by  the  contraction 
of  the  hyoglossus  muscles  as  well  as  the  tongue  itself. 

The  time  required  for  a  mouthful  of  liquid  food  to  pass  to  the  lower  end 
of  the  esophagus  is  approximately  about  o.i  second.  If  the  cardiac  orifice 
is  normally  closed,  a  period  of  about  6  or  7  seconds  may  elapse  before  the 
oncoming  peristaltic  wave  reaches  the  lower  end  of  the  esophagus  and 
forces  the  fluid  into  the  stomach.  If,  however,  a  series  of  deglutitory  acts 
follow  one  another  in  quick  succession  there  is  an  inhibition  of  the  cardiac 
sphincter  and  the  peristaltic  wave,  until  after  the  last  swallow.  The  time 
required  for  the  food  to  pass  into  the  stomach  varies  in  different  animals 
and  in  different  human  beings. 

The  Closure  of  the  Posterior  Nares  and  Larynx.— Because  of  the  rapid 
rise  of  pressure  in  the  pharynx  and  esophagus  during  the  act  of  swallowing 
the  posterior  nares  and  the  opening  of  the  larynx  must  be  closed  to  prevent 
the  food  from  entering  them. 

The  posterior  nares  are  closed  against  the  entrance  of  the  food  by  a 
septum  formed  by  the  pendulous  veil  of  the  palate  and  the  posterior  half 
arches.  The  palate  is  drawn  upward  and  backward  by  the  levator  palati 
muscles,  until  it  meets  the  posterior  wall  of  the  pharynx,  which  at  this 
moment  advances.  At  the  same  time  it  is  made  tense,  by  the  action 
of  the  tensor  palati  muscles.  This  septum  is  completed  by  the  ad- 
vance toward  the  middle  line  of  the  posterior  half  arches  caused  by  the 
contraction  of  the  muscles,  the  palato-pharyngei,  which  compose  them. 
When  these  structures  are  impaired  in  their  functional  activity,  as  in 
diphtheritic  paralysis  and  ulcerations,  there  is  not  infrequently  a  regurgi- 
tation of  food,  especially  liquids,  into  the  nose. 

The  larynx  is  equally  protected  against  the  entrance  of  food  during 
deglutition  under  normal  circumstances.  That  this  accident  occasionally 
happens,  giving  rise  to  severe  spasmodic  coughing,  and  even  in  extreme 
cases  to  suffocation,  is  abundantly  shown  by  the  records  of  cUnical  medi- 
cine. Usually  it  does  not  occur,  for  the  following  reasons:  just  preceding 
and  during  the  act  of  deglutition  there  is  a  complete  suspension  of  the  act 
of  inspiration,  by  which  particles  of  food  might  otherwise  be  drawn  into 
the  larynx;  at  the  same  time  the  larynx  is  always  draW^n  well  up  under  the 
base  of  the  tongue  and  its  entrance  closed  by  the  downward  and  backward 
movement  of  the  epiglottis. 

In  addition  to  the  downward  and  backward  movement  of  the  epiglottis 
and  the  ascent  of  the  larynx  under  the  base  of  the  tongue,  it  is  also  probable 


DIGESTION  67 

that  the  larynx  is  protected  from  the  entrance  of  food,  in  the  rabbit  at 
least,  by  the  closure  of  the  glottis  itself. 

GASTRIC  DIGESTION 

The  Stomach. — Immediately  beyond  the  termination  of  the  esophagus 
the  ahmentary  canal  expands  and  forms  a  receptacle  for  the  temporary  re- 
tention of  the  food.  To  this  dilatation  the  term  stomach  has  been  applied. 
This  organ  is  somewhat  pyriform  in  outline,  and  occupies  the  upper  part  of 
the  abdominal  cavity.  It  is  about  25  to  35  centimeters  long,  15  centi- 
meters deep,  and  10  to  12  centimeters  wide,  and  has  a  capacity  of  about 
1500  c.c.  It  presents  two  orifices,  the  cardiac  or  esophageal,  and  the 
pyloric;  two  curvatures,  the  lesser  and  the  greater. 

The  general  body  of  the  stomach  has  been  divided  into  two  portions, 
viz.,  a  large  portion  to  the  left,  known  as  the  cardiac  portion  and  a  small 
portion  to  right  known  as  the  pyloric  portion.  The  extreme  left  end  of 
the  stomach  is  somewhat  enlarged  and  forms  the  fundus. 

The  stomach  walls  are  formed  by  three  coats: 

1 .  The  serous,  a  reflection  of  the  peritoneum. 

2.  The  muscular,  the  fibers  of  which  are  arranged  in  a  longitudinal,  a 
circular,  and  an  oblique  direction.  In  the  pyloric  portion  of  the  stomach 
the  circular  fibers  increase  enormously  in  number  and  form  thick,  well- 
defined  rings  termed  the  pyloric  muscles.  At  the  pyloric  orifice  the 
muscle  fibers  form  a  distinct  band  termed  the  sphincter  pylori.  The 
orifice  between  the  lower  end  of  the  esophagus  and  stomach  is  also  closed 
by  a  sphincter  known  as  the  sphincter  cardies, 

3.  The  mucous,  which  is  somewhat  larger  than  the  muscular  coat,  and 
in  consequence  is  thrown  into  folds  or  rugae.  The  surface  of  the  mucous 
coat  is  covered  by  tall,  narrow,  columnar  epithelium. 

Gastric  Glands. — Embedded  within  the  mucous  membrane  are  to  be 
found  enormous  numbers  of  tubular  glands,  which  though  resembling  one 
another  in  general  form,  differ  in  their  histologic  details  in  various  portions 
of  the  stomach. 

In  the  cardiac  end  or  fundus,  the  glands  consist  of  several  long  tubules 
opening  into  a  short,  common  duct,  which  opens  by  a  wide  mouth  on  the 
surface  of  the  mucous  membrane.  Each  gland  consists  primarily  of  a 
basement  membrane  lined  by  epithelial  cells.  In  the  duct  the  epithelium 
is  of  the  columnar  variety,  resembhng  that  covering  the  surface  of  the 
mucous  membrane.  The  secretory  portion  of  the  tubule  is  lined  by  a 
layer  of  short,  polyhedral,  granular,  and  nucleated  cells,  which,  as  they 


68 


HUMAN   PHYSIOLOGY 


border  the  lumen  of  the  tubule,  and  thus  occupy  the  central  portion  of 
the  gland,  are  termed  central  cells.  At  irregular  intervals,  between  the 
central  cells  and  the  wall  of  the  tubule,  are  found  large  oval,  reticulated 
cells,  which,  on  account  of  their  position,  are  termed  parietal  cells.  (See 
Fig.  7.) 

Each  parietal  cell  is  in  relation  with  a  system  of  fine  canals,  which  open 
directly  into  the  lumen  of  the  gland.     It  is  estimated  that  the  fundus 


Fig.  7. — Diagram  showing  the  relation  of  the  ultimate  twigs  of  the  blood-vessels, 
V  and  A  and  of  the  absorbent  radicles  to  the  glands  of  the  stomach  and  the  different 
kinds  of  epithelium — viz.,  above  cylindric  cells;  small,  pale  cells  in  the  lumen, 
outside  which  are  the  dark  ovoid  cells. — {Yeo.) 


contains  about  five  million  glands.  In  the  pyloric  end  of  the  stomach 
the  glands  are  generally  branched  at  their  lower  extremities,  and  the 
common  duct  is  long  and  wide.  The  duct  is  lined  by  columnar  epi- 
thelium, while  the  secreting  part  is  lined  by  short,  slightly  columnar, 
granular  cells.  The  parietal  cells  are  entirely  wanting.  The  epithelium 
covering  the  surface  of  the  mucous  membrane  is  tall,  narrow  and  cylindric 
in  shape,  and  consists  of  mucus-secreting  goblet  cells.  The  outer  half  of 
the  cell  contains  a  substance,  mucinogen,  which  produces  mucin.     The 


DIGESTION  69 

gastric  glands  in  both  situations  are  surrounded  by  a  fine  connective 
tissue,  which  supports  blood-vessels,  nerves,  and  lymphatics. 

Changes  in  the  Cells  during  Secretion. — During  the  periods  of  rest 
and  secretory  activity  the  cells  of  the  glands  undergo  changes  in  structure 
which  are  supposed  to  be  connected  with  the  production  of  the  pepsin 
and  hydrochloric  acid.  During  rest,  the  protoplasm  of  the  central  cells 
becomes  filled  with  granular  matter;  during  the  time  of  secretion  this 
disappears,  presumably  passing  into  the  lumen  of  the  tubule,  and  as  a 
result  the  protoplasm  becomes  clear  and  hyalin  in  appearance.  The 
granular  material  is  probably  the  mother  substance,  pepsinogen,  which, 
inactive  in  itself,  yields  the  active  ferment,  pepsin.  The  parietal  cells 
during  digestion  increase  in  size,  but  do  not  become  granular.  It  is  at 
this  period  that  they  secrete  the  hydrochloric  acid.  After  digestion  they 
rapidly  diminish  in  size  and  return  to  their  former  condition.  The  pyloric 
glands  secrete  pepsin  only. 

Gastric  Juice. — The  gastric  juice  obtained  from  the  human  stomach 
free  from  mucus  and  other  impurities  is  a  clear,  colorless  fluid  with  a  con- 
stant acid  reaction,  a  slightly  saline  and  acid  taste,  and  a  specific  gravity 
varying  from  1.002  to  1.005.  When  kept  from  atmospheric  influences, 
it  resists  putrefactive  change  for  a  long  period  of  time,  undergoes  no 
apparent  change  in  composition,  and  loses  none  of  its  digestive  power. 
It  will  also  prevent  and  even  arrest  putrefactive  change  in  organic  matter. 
The  chemic  composition  of  the  gastric  juice  has  never  been  satisfactorily 
determined,  owing  to  the  fact  that  the  secretion  as  obtained  from  fistulous 
openings  has  not  been  absolutely  normal.  It  may  however  be  said  to 
consist  of  water,  organic  matter,  hydrochloric  acid  and  various  inorganic 
salts.  The  quantitative  composition  of  the  juice  varies  somewhat  in 
different  animals. 

The  organic  matter  present  in  gastric  juice  is  a  mixture  of  mucin  and  a 
protein,  products  of  the  metabolic  activity  of  the  epithelial  cells  on  the 
surface  of  the  mucous  membrane  and  of  the  chief  or  central  cells  of  the 
gastric  glands  respectively.  Associated  with  the  protein  material  are 
two  possibly  three  ferment  or  enzyme  bodies,  termed  pepsin,  rennin  and 
lipase.  As  is  the  case  with  other  enzymes,  their  true  chemic  nature  is 
practically  unknown. 

Pepsin. — Pepsin,  though  present  in  gastric  juice,  is  not  present  as  such 
in  the  chief  cells  of  the  glands,  but  is  derived  from  a  zymogen,  propepsin 
or  pepsinogen,  when  the  latter  is  treated  with  hydrochloric  acid.  This 
antecedent  compound  is  related  to  the  granules  observed  in  and  produced 
by  the  cell  protoplasm  during  the  period  of  rest.     Though  pepsin  is 


70  HUMAN  PHYSIOLOGY 

largely  produced  by  the  central  cells  of  the  cardiac  glands,  it  is  also 
produced,  though  in  less  amount,  by  the  cells  of  the  pyloric  glands. 
Pepsin  is  the  chief  proteolytic  or  proteoclastic  agent  of  the  gastric  juice 
and  exerts  its  influence  most  energetically  in  the  presence  of  hydrochloric 
acid  and  at  a  temperature  of  about  4o°C. 

Rennin. — Rennin  or  pexin  is  present  in  the  gastric  juice  not  only  of  man 
and  all  of  the  mammalia,  but  also  of  birds  and  even  fish.  In  its  origin 
from  a  zymogen  substance;  in  its  relation  to  an  acid  medium  and  an 
optimum  temperature  it  bears  a  close  resemblance  to  pepsin.  Its  specific 
action  is  the  coagulation  of  milk,  a  condition  due  to  a  transformation  of 
soluble  caseinogen  into  a  solid  flaky  body,  casein. 

Lipase. — ^Lipase,  an  enzyme  found  in  pancreatic  juice,  has  also  been 
shown  to  be  present  in  gastric  juice,  the  specific  function  of  which  appears 
to  be  the  digestion  of  hydrolysis  of  finely  emulsified  fat  such  as  is  found 
in  milk. 

Hydrochloric  Acid. — Hydrochloric  acid  is  the  agent  which  gives  to  the 
gastric  juice  its  normal  acidity.  Though  the  juice  frequently  contains 
lactic,  acetic,  and  even  phosphoric  acids,  it  is  generally  believed  that 
they  are  the  result  of  fermentation  changes  occurring  in  the  food,  the 
result  of  bacterial  action.  The  percentage  of  hydrochloric  acid  has  been 
the  subject  of  much  discussion.  The  most  recent  investigations  show 
that  the  initial  acidity  of  the  freshly  secreted  human  gastric  juice  is  be- 
tween 0.32  and  0.48  per  cent.  HCl.  This  initial  acidity  is  reduced  by 
combination  with  food,  admixture  with  saliva  and  gastric  mucus,  and  by 
regurgitation  of  alkaline  duodenal  contents,  to  0.15  or  0.2  per  cent.  HCl, 
the  optimum  acidity  for  the  proteolytic  activity  of  pepsin.  As  observed 
clinically,  following  various  test  meals,  the  acidity  of  the  gastric  contents 
is  seen  to  rise  to  a  maximum  as  digestion  progresses,  after  which  it  falls 
to  the  optimum  point  of  about  0.2  per  cent.  HCl. 

Hydrochloric  acid  exerts  its  influence  in  a  variety  of  ways.  It  is  the 
main  agent  in  the  derivation  of  pepsin  and  rennin  or  pexin  from  their 
antecedent  zymogen  compounds,  pepsinogen  and  pexinogen  (Warren); 
it  imparts  activity  to  these  ferments;  it  prevents  and  even  arrests  fer- 
mentative and  putrefactive  changes  in  the  food  by  destroying  micro- 
organisms; it  softens  connective  tissue,  it  dissolves  and  acidifies  the 
proteins,  thus  making  possible  the  subsequent  action  of  pepsin. 

The  inorganic  salts  of  the  gastric  juice  are  probably  only  incidental 
and  play  no  part  in  the  digestive  process. 

Mechanism  of  Secretion. — Modern  investigations  have  established  the 
fact  that  the  production  and  the  discharge  of  gastric  juice  is  under  the 
control  of  a  nerve  center  situated  in  the  medulla  oblongata.    From  this 


DIGESTION  71 

center  nerve  fibers  pass  by  way  of  the  vagus  nerve  to  the  glands  of  the 
stomach.  Division  of  this  nerve  is  followed  by  a  cessation  in  the  flow 
of  the  juice.  Stimulation  of  the  peripheral  end  with  induced  electric 
currents  at  the  rate  of  one  or  two  per  second  causes  the  juice  to  be 
discharged.  Nerve  impulses  therefore,  discharged  by  this  center  descend 
the  vagus  nerve  fibers  to  the  glands  and  excite  them  to  action. 

The  production  and  discharge  of  the  gastric  juice  just  preceding  and 
during  a  meal  is  the  result  of  the  action  of  two  different  stimuli,  a  primary 
and  a  secondary. 

The  primary  stimulus  to  gastric  secretion,  is  a  psychic  state  induced, 
on  the  one  hand,  by  the  sight  or  the  odor  of  food  especially  if  an  indi- 
vidual is  hungry  and  the  food  appetizing;  and  on  the  other  hand  by  the 
mastication  of  food  which  is  agreeable.  The  juice  thus  secreted  is  known 
as  psychic  or  appetite  juice.  The  quantity  of  the  juice  secreted  will  be 
proportional  to  the  agreeable  character  of  the  psychic  state  and  the 
thoroughness  of  mastication.  As  a  result  of  the  psychic  states  nerve 
impulses  descend  nerve  fibers  to  the  center  in  the  medulla  and  excite  it 
to  increased  activity. 

The  secondary  stimulus  to  the  gastric  secretion  is  in  all  probability 
chemic  in  character  and  developed  in  the  stomach  or  in  its  walls  during 
digestive  activity,  inasmuch  as  the  secretion  takes  place  independent  of 
nerve  influences  and  after  division  of  all  afferent  and  efferent  nerves  that 
pass  from  and  to  the  stomach.  The  results  of  experiments  indicate  that 
there  is  produced  in  the  gastric  mucous  membrane  of  the  pyloric  portion 
of  the  stomach  some  chemic  agent,  which  is  absorbed  into  the  blood  and 
carried  to  the  glands  throughout  the  stomach.  On  reaching  the  glands 
this  agent  excites  them  to  continuous  activity.  For  this  reason  the  agent 
has  been  termed  the  gastric  hormone  or  gastric  secretin.  The  stimulus  to 
the  production  of  the  hormone  is  believed  to  be  either  the  action  of  certain 
articles  of  food,  e.g.,  dextrin,  meat  broth  or  the  first  products  of  digestion. 

Physiologic  Action  of  Gastric  Juice. — The  principal  action  of  the  gastric 
juice  is  the  transformation  of  the  different  protein  principles  of  the  food 
into  peptones,  the  intermediate  stages  of  which  are  due  to  the  influence 
of  the  acid  and  pepsin  respectively.  As  soon  as  any  one  of  the  proteins  is 
penetrated  by  the  acid  it  is  converted  into  acid-protein,  a  fact  which  in- 
dicates that  the  first  step  in  gastric  digestion  is  the  acidification  of  the 
proteins.  This  having  been  accomplished,  the  pepsin  becomes  operative 
and  in  a  varying  length  of  time  transforms  the  acid-protein  into  a  new 
form  of  protein  termed  peptone.  In  this  transformation  it  is  possible 
to  isolate  intermediate  bodies  by  the  addition  of  ammonium  and  mag- 
nesium sulphates,  to  which  the  term  proteoses  has  been  given.     Because 


72  -  HUMAN  PHYSIOLOGY 

of  the  order  in  which  they  are  obtained  they  have  been  divided  into  pri- 
mary and  secondary.  This  supposed  change  is  represented  by  the 
following  scheme: 

Protein — Acid-protein — Proteose — Proteose — Peptone 

(primary) — (secondary) 

Peptones. — Peptones  are  the  final  products  of  the  digestion  of  protein 
bodies  in  the  stomach  and  differ  from  the  bodies  from  which  they  are 
derived  in  the  following  particulars: 

1.  They  are  diffusible — i.e.,  capable  of  passing  readily  through  anima 
membranes. 

2.  They  are  soluble  in  water  and  in  saline  solution. 

3.  They  are  non-  coagulable  by  heat  and  nitric  or  acetic  acids.  They  can 
be  readily  precipitated,  however,  by  tannic  acid,  by  bile  acids,  and  by 
mercuric  chlorid. 

The  enzyme  rennin,  causes  the  caseinogen  of  milk  to  undergo  a  peculiar 
change  before  the  acid  and  pepsin  can  convert  it  into  peptone.  This 
change  consists  in  the  cleavage  of  the  caseinogen  into  a  soluble  protein 
and  another  body  which  combining  with  calcium  salts  forms  casein. 
Casein  then  undergoes  a  chemic  transformation  similar  to  that  of  all 
other  proteins. 

The  enzyme  lipase  is  believed  to  digest  fat  when  in  a  finely  emulsified 
state  in  a  manner  similar  to  the  corresponding  enzyme  of  the  pancreatic 
juice. 

Movements  of  the  Stomach. — During  the  period  of  gastric  digestion  the 
walls  of  the  stomach  become  the  seat  of  a  series  of  movements,  somewhat 
peristaltic  in  character,  which  serve  not  only  to  incorporate  the  gastric 
juice  with  the  food,  but  also  serve  to  eject  the  liquefied  portions  of  the  food 
into  the  intestine. 

After  the  entrance  of  the  food  both  the  cardiac  and  pyloric  orifices  are 
closed  by  the  contraction  of  their  sphincters.  Within  five  minutes  (in 
the  cat)  annular  constrictions  begin  in  the  pyloric  region  which  move  peri- 
staltically  toward  the  pylorus.  As  digestion  proceeds  these  constrictions 
or  contractions  become  more  frequent  and  more  vigorous.  The  result  is 
a  trituration  and  liquefaction  of  the  food.  So  soon  as  it  is  liquefied  the 
pylorus  relaxes  and  permits  of  its  discharge  into  the  intestine.  The 
pylorus  then  closes  and  further  preparation  of  food  goes  on.  From  time 
to  time  the  pylorus  relaxes  to  permit  the  discharge  of  prepared  and  lique- 
fied food  until  digestion  is  completed.  The  reason  assigned  for  the  relaxa- 
tion of  the  sphincter  muscle  is  the  presence  of  a  sufficient  amount  of  free 


DIGESTION 


73 


hydrochloric  acid  on  the  gastric  side.  The  reason  assigned  for  its  contrac- 
tion after  the  discharge  of  food  into  the  duodenum  is  the  presence  of  the 
hydrochloric  acid  in  this  region.  With  its  neutralization  by  the  alkalies 
there  present,  its  influence  in  causing  contraction  of  the  sphincter  gradually 
diminishes.  In  the  cardiac  region  there  is  an  absience  of  peristalsis  though 
the  muscle  wall  is  in  a  state  of  active  tone.  The  fundus  acts  as  a  reservoir 
for  food  and  delivers  its  contents  to  the  pyloric  region  as  rapidly  as  it  is 
ready  to  receive  them. 

Nerve  Mechanism  of  the  Stomach. — The  muscle  activities  of  the  walls 
of  the  stomach  as  well  as  the  activities  of  the  sphincter  muscles,  viz., 
the  sphincter  cardiae  and  sphincter  pylori,  are  in  part  inaugurated  and 
modified  from  time  to  time  by  the  nerve  system.  In  a  general  way  it 
may  be  stated  that  the  necessary  muscle  tonus  is  due  to  the  action  of  the 
vagus  nerve.  Division  of  this  nerve  is  followed  by  a  loss  of  tonus.  Stimu- 
lation causes  an  augmentation  in  the  vigor  of  the  contractions  of  the 
pyloric  musculature,  of  the  cardiac  and  fundus  musculature  and  of  the 
sphincters;  an  inhibition  of  these  movements  is  brought  about  by  stimula- 
tion of  the  splanchnic  nerves. 

The  Duration  of  Gastric  Digestion. — The  length  of  time  required  for 
the  digestion  of  a  meal  will  depend  largely  on  the  quantity  and  the  quality 
of  the  foods  consumed.  The  relative  digestibility  of  different  articles  of 
food  was  tested  by  Dr.  Beaumont  on  a  mass  with  a  gastric  fistula.  The 
results  of  his  observation  were  recorded  in  a  table  of  which  the  following 
is  an  abstract. 


Table  Showing  the  Digestibility  of  Various  Articles  of  Food 


Hours      Minutes 


Hours     Minutes 


Eggs,  whipped 

I 

20 

Soup,  barley,  boiled . 

ii 

30 

Eggs,  soft  boiled 

3 

Soup,  bean,  boiled.   . 

3 

Eggs,  hard  boiled.  .  .  . 

3 

30 

Soup,  chicken,  boiled 

3 

Oysters,  raw    

2 

55 

Soup,  mutton,  boiled 

3 

30 

Oysters,  stewed 

1 

30 

Sausage  

3 

20 

Lamb,  broiled 

2 

30 

Green  corn,  boiled    . 

3 

45 

Veal,  broiled    

4 

Beans,  boiled 

2 

30 

Pork,  roasted 

5 

15 

Potatoes,  roasted.   .  . 

2 

30 

Beefsteak,  broiled 

3 

Potatoes,  boiled  .... 

3 

30 

Turkey,  roasted    .... 

2 

25 

Cabbage,  boiled 

4 

30 

Chicken,  boiled 

4 

Turnips,  boiled    .  .  .  . ' 

3 

30 

Chicken,  fricasseed  .  . 

2 

45 

Beets,  boiled    ' 

3 

45 

Duck,  roasted 

4 

._  _j 

Parsnips,  boiled  .... 

2 

30 

74  HUMAN   PHYSIOLOGY 

INTESTINAL  DIGESTION 

The  physical  and  chemic  changes  which  the  food  principles  undergo  in 
the  small  intestine,  and  which  collectively  constitute  intestinal  digestion, 
are  complex  and  probably  more  important  than  those  taking  place  in  the 
stomach,  for  the  food  is,  in  this  situation,  subjected  to  the  solvent  action  of 
the  pancreatic  and  intestinal  juices,  as  well  as  to  the  action  of  the  bile, 
each  of  which  exerts  a  transforming  influence  on  one  or  more  substances 
and  still  further  prepares  them  for  absorption  into  the  blood. 

To  rightly  appreciate  the  physiologic  actions  of  the  digestive  juices 
poured  into  the  intestine,  the  nature  of  the  partially  digested  food  as  it 
comes  from  the  stomach  must  be  kept  in  mind.  This  consists  of  water, 
inorganic  salts,  acidified  proteins,  proteoses,  peptones,  starch,  maltose, 
liquefied  fat,  saccharose,  lactose,  dextrose,  cellulose,  and  the  indigestible 
portion  of  meats,  cereals,  and  fruits.  Collectively  they  are  known  as 
chyme.  As  this  acidified  mass  passes  through  the  duodenum  its  contained 
acids  excite  a  secretion  and  discharge  of  the  intestinal  fluids:  e.g.,  pan- 
creatic juice,  bile,  and  intestinal  juice.  Inasmuch  as  these  fluids  are 
alkaline  in  reaction  they  exert  a  neutralizing  and  precipitating  influence  on 
various  constituents  of  the  chyme.  As  soon  as  this  has  taken  place, 
gastric  digestion  ceases  and  those  chemic  changes  are  inaugurated  which 
eventuate  in  the  transformation  of  all  the  remaining  undigested  nutritive 
materials  into  absorbable  and  assimilable  compounds  which  collectively 
constitute  intestinal  digestion. 

The  Small  Intestine. — This  portion  of  the  alimentary  canal  is  a  convo- 
luted tube,  measuring  about  seven  meters  in  length  and  3.5  cm.  in  diame- 
ter, and  extending  from  the  pyloric  orifice  of  the  stomach  to  the  beginning 
of  the  large  intestine. 

The  Walls  of  the  Small  Intestine. — The  walls  of  the  intestine  consist  of 
four  coats:  viz.,  serous,  muscle,  submucous,  and  mucous. 

1.  The  serous  coat  is  the  most  external  and  is  formed  by  a  reflection  of 
the  general  peritoneal  membrane. 

2.  The  muscle  coat  surrounds  the  entire  intestine  and  consists  of  two 
layers  of  fibers:  i.  an  external  or  longitudinal,  and  2.  an  internal  or  cir- 
cular. The  longitudinal  fibers  form  a  thin  layer  all  over  the  intestine. 
The  circular  fibers  are  much  more  numerous  and  completely  surround 
the  intestine  throughout  its  entire  extent. 

3.  The  mucous  coat  is  soft  and  velvety  and  is  covered  by  a  single  layer  of 
columnar  epithelial  cells.  Its  entire  surface  presents  small  conical  pro- 
jections termed  villi. 


DIGESTION  75 

Blood-vessels,  Nerves,  and  Lymphatics. — The  blood-vessels  of  the  small 
intestine,  which  are  very  numerous,  are  derived  mainly  from  the  su- 
perior mesenteric  artery.  After  penetrating  the  intestinal  walls  the 
smaller  vessels  ramify  in  the  submucous  coat  and  send  branches  to  the 
muscle  and  mucous  coats,  supplying  all  their  structures  with  blood. 
After  circulating  through  the  capillary  vessels  the  blood  is  returned  by 
small  veins  which  subsequently  unite  to  form  the  superior  mesentericv  ein, 
which,  uniting  with  the  splenic  and  gastric  veins,  forms  the  portal  vein. 
The  nerve  elements  in  the  intestinal  wall  consist  of  two  plexuses,  one 
(Auerbach's)  lying  between  the  muscle  coats,  the  other  (Meissner's) 
lying  in  the  submucous  coat.  To  this  nerve  net,  composed  of  nerve  cells 
and  nerve  processes,  found  in  connection  with  the  muscle  coats  of  the 
stomach,  of  the  small  and  of  the  large  intestine  as  well,  the  term  myenteric 
plexus  has  been  given.  The  lymphatics,  which  originate  in  the  mucous 
and  muscle  coats,  are  very  abundant.  They  unite  to  form  those  vessels 
seen  in  the  mesentery  and  empty  into  the  thoracic  duct. 

Intestinal  Glands. — The  gland  apparatus  of  the  intestine  by  which  the 
intestinal  juice  is  secreted  consists  of  the  duodenal  (B runner's)  and  the 
intestinal  (Lieberkiihn's)  glands. 

The  duodenal  glands  are  situated  beneath  the  mucous  membrane  and 
open  by  a  short  wide  duct  on  its  free  surface.  They  are  racemose  glands 
lined  by  nucleated  epithelium.  The  secretion  of  these  glands  is  clear, 
slightly  viscid,  and  alkaline.  Its  chemic  composition  and  functions  are 
unknown. 

The  intestinal  glands  or  follicles  are  distributed  throughout  the  entire 
mucous  membrane  in  enormous  numbers.  They  are  formed  mainly  by 
an  inversion  of  the  mucous  membrane  and  hence  open  on  its  free  surface. 
Each  tubule  consists  of  a  thin  basement  membrane  lined  by  a  layer  of 
spheric  epithelial  cells,  some  of  which  undergo  distention  by  mucin  and 
become  converted  into  mucous  or  goblet  cells.  The  epithelial  secreting 
cells  consist  of  granular  protoplasm  containing  a  well-defined  nucleus. 
The  intestinal  follicles  constitute  the  apparatus  which  secretes  the  chief 
portion  of  the  intestinal  juice. 

The  Pancreas. — This  gland  is  long,  narrow  and  flattened  and  is  situated 
deep  in  the  abdominal  cavity,  lying  just  behind  the  stomach.  It  measures 
from  fifteen  to  twenty  centimeters  in  length,  six  in  breadth,  and  two  and  a 
half  in  thickness.     It  is  usually  divided  into  a  head,  body,  and  tail. 

The  pancreas  communicates  with  the  intestine  by  means  of  a  duct. 
This  duct  commences  at  the  tail  and  runs  transversely  through  the  body 
of  the  gland.    As  it  approaches  the  head  of  the  gland  it  gradually  increases 


76 


HUMAN   PHYSIOLOGY 


in  size  until  it  measures  about  two  or  three  millimeters  in  diameter.  It 
then  curv'es  downward  and  forward  and  opens  into  the  duodenum.  In 
its  course  through  the  gland  it  rece'ves  branches  which  enter  it  nearly  at 
right  angles. 

The  pancreas  is  similar  in  structure  to  the  salivary  glands,  and  con- 
sists of  the  system  of  ducts  terminating  in  acini.  The  acini  are  tubular 
or  flask-shaped,  and  consist  of  a  basement  membrane  lined  by  a  layer 
of  cylindric,  conic  cells,  which  encroach  upon  the  lumen  of  the  acini. 
The  cells  exhibit  a  difference  in  their  structure  (Fig.  8),  and  may  be  said 
to  consist  of  two  zones — viz.,  an  outer  parietal  zone,  which  is  transparent 


Fig.  8. — One  Saccule  of  the  Pancreas  of  the  Rabbit  in  Different  States 
OF  Activity. — {After  Kuhne  and  Lea.) 
A.  After  a  period  of  rest,  in  which  case  the  outlines  of  the  cells  are  indistinct  and 
the  inner  zone — i.e.,  the  part  of  the  cells  (a)  next  the  lumen  (c) — is  broad  and  filled 
with  fine  granules.  B.  After  the  gland  has  poured  out  its  secretion,  when  the  cell 
outlines  (rf)  are  clearer,  the  granular  zone  (a)  is  smaller,  and  the  clear  outer  zone  is 
wider. 


and  apparently  homogeneous,  staining  rapidly  with  carmin;  an  inner 
zone,  which  borders  the  lumen,  and  is  distinctly  granular  and  stains  but 
slightly  with  carmin.  These  cells  undergo  changes  similar  to  those 
exhibited  by  the  cells  of  the  salivary  glands  during  and  after  active  secre- 
tion. As  soon  as  the  secretory  activity  of  the  pancreas  is  establislied,  the 
granules  disappear,  and  the  inner  granular  layer  becomes  reduced  to  a 
very  narrow  border,  while  the  outer  zone  increases  in  size  and  occupies 
nearly  the  entire  cell.  During  the  intervals  of  secretion,  however,  the 
granular  layer  reappears  and  increases  in  size  until  the  outer  zone  is  re- 
duced to  a  minimum.  It  would  seem  that  the  granular  matter  is  formed 
by  the  nutritive  processes  occurring  in  the  gland  during  rest,  and  is  dis- 
charged during  secretory  activity  into  the  ducts,  and  takes  part  in  the 
formation  of  the  pancreatic  secretion. 


DIGESTION  77 

Toward  the  outer  extremity  of  the  pancreas  there  are  found  among  the 
acini  collections  of  globular  cells  arranged  in  rods  or  columns  separated 
by  connective  tissue.  They  have  been  termed  after  their  discoverer,  the 
Islands  of  Langerhans.  It  is  believed  they  produce  an  internal  secretion 
which  in  some  way  regulates  sugar  metabolism. 

The  Pancreatic  Juice. — The  pancreatic  juice  is  transparent,  colorless, 
strongly  alkaline,  and  viscid,  and  has  a  specific  gravity  of  1,020.  It  is 
one  of  the  most  important  of  the  digestive  fluids,  as  it  exerts  a  transform- 
ing influence  upon  all  classes  of  alimentary  principles,  and  has  been 
shown  to  contain  at  least  three  distinct  enzymes,  viz.,  amylopsin,  trypsin, 
steapsin  or  lipase.     It  has  the  following  composition : 

Composition  of  Pancreatic  Juice 

Water 900 ,  76 

Protein  material    90 .  44 

Inorganic  salts 8 .  80 


Mode  of  Secretion. — The  secretion  and  discharge  of  the  pancreatic 
juice  is  associated  with  the  introduction  of  food  into  the  mouth  and 
stomach  and  its  early  passage  into  the  duodenum  and  is  brought  about 
by  the  action  of  a  primary  and  a  secondary  stimulus. 

The  primary  stimulus  is  a  psychic  state  according  to  Pavlov  induced 
by  the  sight,  odor  and  taste  of  food  and  which  leads  to  the  discharge  of 
nerve  impulses  from  nerve-cells  in  the  medulla  oblongata  and  their  trans- 
mission by  efferent  nerves  in  the  trunk  of  the  vagus  nerve,  to  the  cells  of 
the  acini.  It  is  probable  that  the  impressions  made  by  the  food  on  the 
criminal  filaments  of  the  afferent  fibers  in  the  vagus  nerve  develop  nerve 
impulses  which,  when  transmitted  to  the  medulla,  occasion  the  discharge 
of  nerve  impulses  that  not  only  excite  the  secretion  but  increase  the  blood- 
supply  as  well. 

The  secondary  stimulus  is  chemic  in  character  and  developed  in  the 
glands  of  the  mucous  membrane  of  the  duodenum  by  the  action  of  the 
acids  of  the  chyme,  that  is,  of  the  digested  foods,  coming  through  the 
pylorus. 

If  an  extract  of  the  glandular  portion  of  the  duodenal  mucous  membrane, 
made  with  hydrochloric  acid  0.4  per  cent,  is  injected  into  the  blood  it 
evokes  a  profuse  discharge  of  pancreatic  juice.  As  hydrochloric  acid 
alone  will  not  produce  this  effect  it  is  assumed  that  the  extract  contains 
an  agent  that  excites  or  arouses  the  pancreas  to  secretor  activity  and 
to  which,  therefore,  the  name  secretin  is  given.     The  secretin  developed 


78  HUMAN  PHYSIOLOGY 

by  the  passage  of  the  acid  food  over  the  surface  of  the  mucous  membrane 
is  absorbed  into  the  blood  and  carried  eventually  to  the  pancreas  and 
brought  into  relation  with  the  cells  on  which  it  exerts  its  stimulating 
action.    This  agent  belongs  to  the  class  of  hormones. 

Physiologic  Action. — By  virtue  of  its  contained  enzymes  pancreatic 
juice  acts  on : 

1.  On  Starch, — When  normal  pancreatic  juice  or  a  glycerin  extract  of 
the  gland  is  added  to  a  solution  of  hydrated  starch,  the  latter  is  speedily 
transformed  into  maltose,  passing  through  the  intermediate  stage  of 
dextrin.  The  process  is  in  all  respects  similar  to  that  observed  in  the 
digestion  of  starch  by  saliva.  Pancreatic  juice,  however,  is  more  energetic 
in  this  respect  than  saliva.  The  enzyme  which  effects  this  change  is 
termed  amylopsin.  When  the  starch  which  escapes  salivary  digestion 
passes  into  the  small  intestine  and  mingles  with  pancreatic  juice,  it  is 
very  promptly  converted  into  maltose  by  the  action  or  in  the  presence 
of  this  enzyme. 

2.  On  Protein. — The  protein  bodies  which  escape  digestion  in  the 
stomach  are  converted  into  peptones  by  the  action  of  the  alkali  and 
ferment.  The  first  effect  of  the  alkali  is  to  change  the  protein  into  an 
alkali-protein,  a  fact  which  indicates  that  in  the  digestion  of  protein  by 
pancreatic  juice,  the  first  stage  is  alkalinization.  This  having  been 
accomplished,  the  ferment  trypsin  transforms  the  alkali-albumin  into 
peptone.  The  addition  of  magnesium  sulphate  to  the  digestion  mixtures 
causes  a  precipitation  of  an  intermediate  termed  proteose.  For  this 
reason  it  is  believed  that  here  also  peptones  are  preceded  in  their  develop- 
ment by  proteoses,  of  which  there  is  probably,  however,  but  one  form, 
viz.,  secondary  proteoses.  Long-continued  action  of  the  pancreatic  juice, 
decomposes  the  peptone  into  leucin,  tyrosin,  histidin  aspartic  acid,  etc., 
compounds  which  belong  to  the  group  of  bodies  known  as  antfifio-acids, 
etc. 

3.  On  Fat. — If  pancreatic  juice  be  added  to  a  perfectly  neutral  fat — 
olein,  palmitin,  or  stearin — and  kept  at  a  temperature  of  about  ioo°F. 
(38°C.),  it  will  at  the  end  of  an  hour  or  two  be  partially  decomposed  into 
glycerin  and  the  particular  fat  acid  indicated  by  the  name  of  the  fat 
used — e.g.,  oleic,  palmitic,  stearic.  The  oil  will  then  exhibit  an  acid 
reaction.     The  reaction  is  represented  in  the  following  formula: 

C3H5(Ci8H3302)3     +      3H2O      =      Ci8H3402     +      C3H6(HO)3 

Triolein.  Water.  Oleic  Acid.  Glycerin 

If  to  this  acidified  oil  there  be  added  an  alkali,  e.g.,  potassium  or 
sodium  carbonate,  the  latter  will  at  once  combine  with  the  fat  acid  to 


DIGESTION  79 

form  a  salt  known  as  a  soap.  The  reaction  is  expressed  in  the  follow- 
ing equation: 

NasCOa      +      C18H34O2      =      2NaCi8H3302      +      H2CO3 

Sodium  Carbonate.  Oleic  Acid.  Sodium  Oleate.  Carbonic  Acid. 

Coincident  with  the  formation  of  the  soap,  the  remaining  portion  of  the 
neutral  oil  will  undergo  division  into  globules  of  microscopic  size,  which 
are  held  in  suspension  in  the  soap  solution,  forming  what  has  been  termed 
an  emulsion,  which  is  white  and  creamy  in  appearance.  The  cause  of 
this  minute  subdivision  of  the  fat  and  the  necessity  for  it  is  unknown. 
It  may  be  assumed  that  by  virtue  of  the  subdivision  a  greater  surface  is 
exposed  to  the  action  of  the  pancreatic  enzyme  and  the  digestion  of  the 
fat  thereby  facilitated.  The  action  of  the  pancreatic  juice  may  then  be 
said  to  consist  in  the  cleavage  of  the  neutral  fats  into  fatty  acids  and 
glycerin,  after  which  the  formation  of  the  soap  and  the  division  of  the 
fat  takes  place  spontaneously.  The  enzyme  which  produces  the  cleavage 
of  the  neutral  fats  has  been  termed  steapsin  or  lipase. 

Physiologic  Action  of  Intestinal  Juice. — By  reason  of  its  contained  en- 
zymes intestinal  juice  acts: 

1.  On  Proteoses  and  Peptones. — These  bodies  were  supposed  at  one  time 
to  represent  the  final  stages  in  the  digestion  of  the  proteins.  This  view 
is  no  longer  entertained.  It  is  now  generally  believed  that  under  the 
influence  of  the  intestinal  juice  they  undergo  a  disruption  into  very  simple 
bodies,  known  as  amino-acids.  This  disruption  is  brought  about  by  an 
agent  termed  erepsin.  Inasmuch  as  the  long-continued  action  of  pan- 
creatic juice  also  disrupts  the  peptones,  it  is  also  believed  to  contain 
erepsin. 

2.  On  The  Compound  Sugars. — Saccharose,  maltose  and  lactose,  the 
three  compound  sngars,  are  believed  by  most  observers  to  be  not  only  non- 
absorbable, but  also  non-assimilable  and,  therefore,  are  required  to  under- 
go some  digestive  change  before  they  can  be  absorbed  and  assimilated. 
An  extract  of  the  intestinal  mucous  membrane  or  the  intestinal  juice 
of  the  dog  added  to  a  solution  of  saccharose  will  cause  it  to  combine 
chemically  with  water  after  which  a  cleavage  into  dextrose  and  levulose 
will  take  place,  which  together  constitute  invert  sugar.  The  enzyme 
to  which  this  action  is  attributed  has  been  termed  invertase  of  saccharase. 
Maltose  undergoes  a  similar  change.  After  its  combination  with  water 
it  undergoes  a  cleavage  into  two  molecules  of  dextrose.  Lactose  appears 
to  be  unaffected  by  the  pure  juice.  As  it  is  non-assimilable  it  has  been 
supposed  to  undergo  conversion  into  dextrose  and  galactose  while  passing 
through  the  epithelial  cells  of  the  intestinal  mucosa.     In  either  case  the 


80  HUMAN  PHYSIOLOGY 

transfonnation  is  brought  about  by  two  ferments  known  respectively  as 
maltose  and  lactase. 

3.  On  Trypsinogen. — This  zymogen  when  first  discharged  from  the  pan- 
creatic duct  is  inactive  and  incapable  of  effecting  the  necessary  digestive 
changes  in  the  proteins.  Shortly  after  its  entrance  into  the  intestine,  it 
becomes  quite  active  and  efficient,  a  change  attributed  to  an  agent 
entero-kinase  secreted  by  the  mucosa  in  the  upper  part  of  the  intestine. 

The  Bile. — This  fluid  is  a  product  of  the  secretor  activity  of  the  liver- 
cells.  After  its  formation  by  the  liver  cells  it  is  conveyed  from  the  liver 
by  the  bile  capillaries  which  unite  finally  to  form  the  main  hepatic  duct. 
This  duct  emerges  from  the  liver  at  the  transverse  fissure.  At  a  distance 
of  about  5  centimeters  it  is  joined  by  the  cystic  duct,  the  distal  extremity 
of  which  expands  into  a  pear-shaped  reservoir,  the  gall-bladder  in  which 
the  bile  is  temporarily  stored.  The  duct  formed  by  the  union  of  the 
hepatic  and  cystic  ducts,  the  common  bile-duct,  passes  downward  and 
forward  for  a  distance  of  about  7  centimeters,  pierces  the  walls  of  the 
intestine  and  passes  obliquely  through  its  coats  for  about  a  centimeter  and 
opens  into  a  small  receptacle,  the  ampulla  of  Vater. 

Physical  Properties. — The  bile  coming  from  the  liver  is  thin  and  watery. 
That  obtained  from  the  gall-bladder  is  more  or  less  viscid  from  the  presence 
of  mucin.  The  specific  gravity  of  human  bile  varies  within  normal 
limits  from  i.oio  to  1.020.  The  reaction  is  invariably  alkaline  in  the 
human  subject  when  first  discharged  from  the  liver,  but  may  become 
neutral  in  the  gall-bladder.  The  alkalinity  depends  on  the  presence  of 
sodium  carbonate  and  sodium  phosphate.  When  fresh,  it  is  inodorous; 
but  it  readily  undergoes  putrefactive  changes,  and  soon  becomes  offensive. 
Its  taste  is  decidedly  bitter.  When  shaken  with  water,  it  becomes  frothy 
— a  condition  which  lasts  for  some  time  and  which  is  due  to  the  presence 
of  mucin.     In  ox  bile  the  mucin  is  replaced  by  a  nucleo-protein. 

The  color  of  bile  obtained  from  the  hepatic  duct  is  variable,  usually  a 
shade  between  a  greenish  yeUow  and  a  brownish  red.  In  different 
animals  the  color  varies.  In  the  herbivorous  animals  it  is  usually  green; 
in  the  carnivorous  animals  it  is  orange  or  brown.  In  man  it  is  green  or  a 
golden  yellow.  The  colors  are  due  to  the  presence  of  pigments.  Micro- 
scopic examination  fails  to  show  the  presence  of  structural  elements. 

Chemic  Composition. — Human  bile  obtained  from  an  accidental  biliary 
fistula  was  shown  by  Jacobson  to  contain  the  following  ingredients,  viz. : 


DIGESTION  8 1 
Composition  of  Human  Bile 

Water 977-40 

Sodium  glycocholate 9-94 

Sodium  taurocholate a  trace 

Cholesterol o .  54 

Free  fat o .  lo 

Sodium  palmitate  and  stearate i .  36 

Lecithin 0 .  04 

Organic  matter,  and  pigments  bilirubin  and  biliverdin    2  .  26 

Inorganic  salts 8  .36 


1,000.00 


Sodium  glycocholate  and  sodium  taurocholate  are  the  characteristic 
biliary  salts.  They  are  compounds  of  sodium  and  glycocholic  and  tauro- 
cholic  acids.  There  is  evidence  that  the  former  is  formed  by  the  union  of 
an  amino-acid  glycocoll  and  cholic  acid,  and  that  the  latter  is  formed 
by  the  union  of  taurin,  a  derivative  of  the  amino-acid  cystin,  both  of 
which  are  absorbed  from  the  intestines.  The  origin  of  cholic  acid  is 
not  clear. 

There  is  good  evidence  for  the  view,  that  after  their  discharge  into 
the  intestine,  the  bile  salts  are  absorbed,  with  the  exception  of  a  portion 
destroyed  by  bacteria,  and  carried  by  the  portal  vein  to  the  liver  and  again 
excreted.  By  this  circulation  from  liver  to  intestine  and  from  intestine  to 
the  liver,  the  work  of  the  liver  cells  in  the  synthesis  or  secretion  of  bile 
acids,  is  supposed  to  be  reduced  to  a  minimum.  It  is  also  probable  that  a 
portion  of  the  acids  enters  the  general  circulation  and  influences  favorably 
the  general  nutrition.  It  is  stated  by  some  investigators  that  the  activities 
of  the  liver  cells  are  decidedly  increased  by  the  circulation  of  the  bile  salts 
and  that  they  are  to  be  regarded  as  the  natural  stimuli  to  the  secretion. 

Cholesterol. — Cholesterol  when  obtained  from  bile  presents  itself  in  the 
form  of  flat  rectangular  crystals.  Though  a  constant  constituent  of 
bile,  is  not  confined  to  this  fluid  as  it  has  been  shown  to  be  a  normal  con- 
stituent of  all  animal  and  vegetable  ceUs,  though  it  is  particularly  abun- 
dant in  the  myelin  of  nerve-fibers.  Though  cholesterol  has  for  a  long 
time  been  regarded  merely  as  one  of  the  products  of  the  katabolism  of 
living  material,  it  has  come  to  be  believed  that  it  is  necessary  to  the 
vitality  of  tissue  cells  and  especially  to  the  blood  cells.  Entering  into 
the  composition  of  the  surface  layer  of  cells,  it  prevents  the  entrance  of 
certain  toxins  which  would  have  a  destructive  influence  on  their  structure 
or  composition.  In  the  metabolism  of  cells  it  is  set  free  after  which  it 
passes  into  the  blood  to  be  secreted  by  the  liver.  In  the  bile  it  frequently 
undergoes  crystallization  and  forms  one  of  the  forms  of  gall-stones.  In 
6 


S2  HUMAN   PHYSIOLOGY 

the  bile  the  cholesterol  is  held  in  solution  by  the  biliary  salts.    In  the 
intestine  it  is  converted  into  stercorin  and  discharged  in  the  feces. 

Bilirubin^  Biliverdln. — These  two  pigments  impart  to  the  bile  its 
red  and  green  colors  respectively.  Bilirubin  is  present  in  the  bile  of  human 
beings  and  the  carnivora,  biliverdin  in  the  bile  of  the  herbivora.  As  the 
former  pigment  readily  undergoes  oxidation  in  the  gall-bladder,  giving  rise 
to  the  latter  pigment,  almost  any  specimen  of  bile  may  present  any  shade 
of  color  between  red  and  green.  Bilirubin  is  regarded  as  a  derivative  of 
hematin,  one  of  the  cleavage  products  of  hemoglobin,  the  coloring-matter 
of  the  blood.  In  the  liver  the  hematin  combines  with  water,  loses  its^iron, 
and  is  changed  to  bilibrubin.  By  continuous  oxidation  there  are  formed 
biliverdin,  bilicyanin,  and  choletelin.  After  their  discharge  into  the  in- 
testine the  bile  pigments  are  finally  reduced  to  hydrobilirubin  or  an  allied 
substance,  stercobilin,  which  becomes  one  of  the  constituents  of  the  feces. 
A  portion  of  the  latter  is  absorbed  into  the  blood  and  ultimately  discharged 
into  the  urine  where  it  is  known  as  urobilin. 

Lecithin. — ^Lecithin  is  regarded,  because  of  its  physical  properties  and 
chemic  composition,  as  a  complex  fat.  When  pure  it  presents  itself 
generally  as  a  white  crystalline  powder,  though  very  frequently  as  a  white 
waxy  mass  which  is  soluble  in  ether  and  alcohol.  Lecithin  is  widely  dis- 
tributed throughout  the  body,  being  found  in  blood,  lympth,  red  and 
white  corpuscles,  nerve-tissue,  yolk  of  egg,  semen,  milk,  and  bile.  Lecithin 
has  been  regarded  as  one  of  the  decomposition  products  of  nerve-tissue, 
removed  from  the  blood  by  the  liver  and  thus  becoming  one  of  the  con- 
stituents of  the  bile,  in  which  it  is  held  in  solution  by  the  bile  salts. 
Lecithin  can  be  readily  decomposed  by  various  agents  yielding  gly- 
cophosphoric  acid,  a  fat  acid  and  cholin. 

The  Mode  of  Secretion  and  Discharge  of  Bile. — The  flow  of  bile  from 
the  liver  is  continuous  but  subject  to  considerable  variation  during  the 
twenty-four  hours.  The  introduction  of  food  into  the  stomach  at  once 
causes  a  slight  increase  in  the  flow,  but  it  is  not  until  about  two  hours  later 
that  the  amount  discharged  reaches  its  maximum;  after  this  period  it 
gradually  decreases  up  to  the  eighth  hour,  but  never  entirely  ceases. 
During  the  intervals  of  digestion  though  a  small  quantity  passes  into  the 
intestine,  the  main  portion  is  diverted  into  the  gall-bladder,  because  of 
the  closure  of  the  common  bile-duct  by  the  sphincter  muscle  near  its 
termination,  where  it  is  retained  until  required  for  digestive  purposes. 
When  acidulated  food  passes  over  the  surface  of  the  duodenum,  there  is  an 
increase  in  the  secretion  or  at  least  the  discharge  of  bile,  due  to  the  relaxa- 
tion of  the  sphincter  muscle  of  the  common  bile-duct  and  the  contraction 
of  the  muscle  walls  of  the  gall-bladder  and  biliary  passages. 


DIGESTION  83 

Physiologic  Action  of  Bile. — The  exact  relation  of  the  bile  to  the  digest- 
ive process  has  not  been  satisfactorily  determined.  No  specific  action 
can  be  attributed  to  it.  It  has  but 'a  slight,  if  any,  diastatic  action  on 
starch.  It  is  without  influence  on  proteins  or  on  fats  directly.  But 
indirectly  and  by  virtue  of  tiie  bile  salts  it  contains,  it  plays  an  important 
part  in  increasing  the  action  of  the  pancreatic  enzymes.  Thus  the  amylo- 
ly tic  or  starch  transforming  power  of  the  pancreatic  juice  is  almost  doubled 
and  the  same  is  true  for  its  proteolytic  power,  while  its  lipolytic  or  fat- 
splitting  power  is  tripled. 

The  bile  salts  also  dissolve  insoluble  soaps  which  may  be  formed  during 
digestion  and  thus  favors  the  digestion  of  fat.  If  it  be  excluded  from  the 
intestine  there  is  found  in  the  feces  from  22  to  58  per  cent,  of  the  ingested 
fats.  At  the  same  time  the  chyle,  instead  of  presenting  the  usual  white 
creamy  appearance,  is  thin  and  slightly  yellow.  The  manner  in  which  the 
bile  promotes  fat  digestion  is  yet  a  subject  of  investigation.  If  all  the  fat 
is  converted  into  fat  acids  and  glycerin,  with  the  formation  of  soaps,  as 
seems  probable,  the  action  of  the  bile  becomes  more  apparent  from  the  fact, 
already  stated,  that  it  dissolves  and  holds  in  solution  the  soaps  so  formed 
which  would  be  necessary  to  their  absorption  by  the  epithelial  cells.  This 
action  has  been  attributed  to  the  presence  of  the  bile  salts.  As  an  aid  to 
digestion  the  bile  has  been  regarded  as  important,  for  the  reason  that  its 
entrance  into  the  intestine  is  attended  by  a  neutralization  and  precipita- 
tion of  the  proteins  which  have  not  been  fully  digested  and  are  yet  in  the 
stage  of  acid-albumin.  In  this  way  gastric  digestion  is  arrested  and  the 
foods  are  prepared  for  intestinal  digestion. 

Though  bile  possesses  no  antiseptic  properties  outside  the  body,  itself 
undergoing  putrefactive  changes  very  rapidly,  it  has  been  believed  that  in 
the  intestine  it  in  some  way  prevents  or  retards  putrefactive  changes  in  the 
food.  There  can  be  no  doubt  that  if  the  bile  is  prevented  from  entering 
the  intestine  there  is  an  increase  in  the  formation  of  gases  and  other  prod- 
ucts which  impart  to  the  feces  certain  characteristics  which  are  indicative 
of  putrefaction.  As  to  the  manner  in  which  bile  retards  this  process 
nothing  definite  can  be  stated.  It  has  been  supposed  to  be  a  stimulant 
to  the  peristaltic  movements  of  the  intestine,  inasmuch  as  these  move- 
ments diminish  when  bile  is  diverted  from  the  intestine. 

Intestinal  Movements. — During  intestinal  digestion  the  walls  of  the 
intestine  exhibit  two  kinds  of  movement,  viz.,  a  rhythmic  segmentation 
and  a  peristalsis.  By  the  former  the  food  is  divided  into  segments  and 
by  the  latter,  it  is  carried  down  the  intestine.  Shortly  after  the  entrance 
of  food  into  the  duodenum  a  broad  peristaltic  wave  promptly  carries  it 
downward  a  variable  distance  a  rhythmic  segmentation  begins  by  a  con- 


84  HUMAN   PHYSIOLOGY 

traction  of  bands  of  circular  muscle  fibers.  So  soon  as  a  mass  of  food  is 
divided  into  segments  each  segment  is  in  turn  again  divided  by  similar 
contractions.  The  lower  half  of  each  segment  then  unites  with  the  upper 
half  of  the  segment  below  to  commingle  with  it  and  to  expose  new  surfaces 
of  the  food  mass  to  contact  with  the  intestinal  juices  and  to  the  mucous 
membrane.  A  continual  repetition  of  this  process  results  in  a  thorough 
mixing  of  the  food  with  the  digestive  juices.  Subsequent  peristaltic 
waves  slowly  carry  the  food  further  down  the  intestine,  after  which  a 
further  segmentation  takes  place.  These  alternate  movements  continue 
throughout  the  digestive  process. 

The  Nerve  Mechanism  of  the  Small  Intestine. — The  rhythmic  segmen- 
tation movements  are  the  result  of  an  in  train  testinal  pressure  due  to  the 
accumulation  of  food,  provided  the  intestinal  walls  possess  the  requisite 
degree  of  tonicity.  The  tonicity  is  imparted  to  the  muscle  coat  by  nerve 
impulses  coming  from  the  central  nerve  system  through  the  efferent  vagus 
nerve  fibers.  The  orderly  and  coordinated  contractions  and  relaxations 
of  the  muscle  coat  which  constitute  a  peristaltic  movement  are  mediated 
by  the  myenteric  plexus — the  nerve  plexus  of  Meissner  and  Auerbach — 
and  therefore  termed  a  myenteric  reflex. 

The  intestine  is  connected  with  the  central  nerve  system  by  the  vagi 
and  splanchnic  nerves,  both  of  which  influence  the  tonus  and  the  vigor 
of  the  intestinal  contractions  in  one  direction  or  the  other.  Thus  stimula- 
tion of  the  vagus  nerve  increases  the  contractions,  while  stimulation  of 
the  splanchnic  inhibits  the  contractions.  The  degree  of  activity  of  the 
intestine  at  any  one  moment  is  the  resultant  of  the  opposing  actions  of 
these  two  nerves. 

The  Large  Intestine. — The  large  intestine  is  that  portion  of  the  ali- 
mentary canal  situated  between  the  termination  of  the  ileum  and  the 
anus.  It  varies  in  length  from  one  and  a  quarter  to  one  and  a  half  meters, 
in  diameter  from  three  and  a  half  to  seven  centimeters.  It  is  divided  into 
the  cecum,  the  colon  (subdivided  into  an  ascending,  transverse,  and 
descending  portion,   including   the   sigmoid   flexure),   and   the   rectum. 

The  walls  of  the  large  intestine  consist  of  three  coats:  viz.,  serous, 
muscular,  and  mucous. 

The  serous  is  a  reflection  of  the  general  peritoneal  membrane. 

The  muscle  is  composed  of  both  longitudinal  and  circular  fibers.  The 
longitudinal  fibers  are  collected  into  three  narrow  bands  which  are  situated 
at  points  equidistant  from  one  another.  At  the  rectum  they  spread  out 
so  as  to  surround  it  completely.  As  the  longitudinal  bands  are  shorter 
than  the  intestine  itself,  its  surface  becomes  sacculated,  each  sac  being 


DIGESTION  85 

partially  separated  from  adjoining  sacs  by  narrow  constrictions.  The  cir- 
cular fibers  are  arranged  in  the  form  of  a  thin  layer  over  the  entire  intes- 
tine. Between  the  sacculi,  however,  they  are  more  closely  arranged.  The 
sacculi  have  been  termed  haustra  from  their  supposed  function,  that  of 
absorbing  or  drawing  water  from  the  intestinal  contents  thus  imparting 
to  them  a  certain  degree  of  consistency.  In  the  rectum  the  circular  fibers 
are  well  developed,  and  at  a  point  an  inch  above  the  anus  they  form,  as 
stated  above,  the  internal  sphincter. 

The  mucous  membrane  of  the  large  intestine  possesses  neither  villi  nor 
valvulae  conhiventes.  It  contains  a  large  number  of  tubules  consisting 
of  a  basement  membrane  lined  by  columnar  epithelium.  They  resemble 
the  follicles  of  Lieberkiihn.  The  secretion  of  these  glands  is  thick  and 
viscid  and  contains  a  large  quantity  of  mucin. 

The  Movements  of  the  Large  Intestine. — After  the  absorption  of  the 
prepared  food  materials,  the  remaining  contents  of  the  intestine,  together 
with  certain  intestinal  secretions  and  the  excrementitious  matter  of  the 
bile,  pass  into  the  large  intestine  and  assist  in  the  formation  of  the  feces. 

Under  the  influence  of  a  peristaltic  movement  similar  to  that  wit- 
nessed in  the  small  intestine,  all  this  excrementitious  matter,  deprived  by 
absorption  of  the  excess  of  its  contained  water  and  nutritive  material,  is 
gradually  carried  downward  to  the  sigmoid  flexure,  where  it  accumulates 
prior  to  its  extrusion  from  the  body.  The  effects  of  the  peristaltic  waves 
are  to  some  extent  interfered  with  by  anti- peristaltic  or  anastaltic  waves 
which,  beginning  in  the  transverse  colon,  run  toward  and  to  the  cecum. 
An  antiperistaltic  wave  occurs  in  the  cat  about  every  fifteen  minutes  and 
lasts  for  about  five  minutes.  The  intestinal  contents  are  thereby  driven 
back  toward  the  cecum.  The  effect  is  a  still  further  admixture  with  the 
secretions  and  exposure  to  the  absorbing  mucosa. 

In  addition  to  the  anastaltic  waves,  contractions  of  the  haustra  have 
been  observed  which  resemble  the  segmentation  contractions  observed  in 
the  small  intestine  and  which  promote  it  is  believed  the  absorption  of 
water  from  the  intestinal  contents. 

The  Function  of  the  Large  Intestine. — The  function  of  the  large  intes- 
tine is  therefore  to  receive,  to  reduce  to  a  proper  consistency,  to  tem- 
porarily store  and  subsequently  discharge  its  contents,  consisting  of  the 
indigestible  residue  of  the  food,  together  with  excretions  of  intestinal 
glands  which  have  descended  from  the  small  intestine  and  which  constitute 
in  part  the  fec.es. 

Intestinal  Fermentation. — Owing  to  the  favorable  conditions  in  both 
the  small  and  large  intestine  for  fermentative  and  putrefactive  processes — 


86  HUMAN   PHYSIOLOGY 

e.g.y  heat,  moisture,  oxygen,  and  the  presence  of  various  microorganisms — 
the  food,  when  consumed  in  excessive  quantity  of  when  acted  on  by  defect- 
ive secretions,  undergoes  a  series  of  decomposition  changes  which  are 
attended  by  the  production  of  gases  and  various  chemic  compounds. 
Among  the  more  important  of  these  compounds  may  be  mentioned  indol, 
skatol,  cresol  and  phenol.  They  arise  from  the  putrefactive  decomposi- 
tion of  various  amino-acids.  A  certain  portion  of  each  is  eliminated  in 
the  feces  while  another  portion  is  absorbed  into  the  portal  circulation, 
oxidized,  and  combined  with  potassium  sulphate.  By  this  means  their 
toxicity  is  destroyed.  They  are  subsequently  eliminated  in  the  urine. 
The  amount  of  the  potassium  indoxyl  sulphate  or  indican  in  the  urine  is 
taken  as  a  meature  of  the  extent  of  intestinal  putrefaction. 

The  Feces. — The  feces  is  a  term  applied  to  the  mass  of  material  ejected 
from  the  rectum  through  the  anus.  They  are  characterized  by  consist- 
ency, color  and  odor,  properties  which  are  connected  with  the  rapidity 
with  which  they  are  carried  through  the  intestine,  the  quality  of  the  food, 
and  the  extent  of  the  fermentative  and  putrefactive  changes  they  undergo. 

Defecation. — Defecation  is  the  voluntary  act  of  extruding  the  feces 
from  the  rectum,  and  is  accomplished  by  a  relaxation  of  the  sphincter  ani 
muscle  and  by  the  contraction  of  the  muscular  walls  of  the  rectum,  aided 
by  the  contraction  of  the  abdominal  muscles. 

ABSORPTION 

The  term  absorption  is  applied  to  the  passage  or  transference  of 
materials  into  the  blood  from  the  tissues,  from  the  serous  cavities,  and  from 
the  mucous  surfaces  of  the  body.  The  most  important  of  these  surfaces, 
especially  in  its  relation  to  the  formation  of  blood,  is  the  mucous  surface 
of  the  alimentary  canal;  for  it  is  from  this  organ  that  new  materials  are 
derived  which  maintain  the  quality  and  quantity  of  the  blood.  The  ab- 
sorption of  materials  from  the  interstices  of  the  tissues  is  to  be  regarded 
rather  as  a  return  to  the  blood  of  liquid  nutritive  material  which  has  es- 
caped from  the  blood-vessels  for  nutritive  purposes,  and  which,  if  not 
returned,  would  lead  to  an  accumulation  of  such  fluid  and  the  development 
of  dropsical  conditions. 

The  anatomic  mechanisms  involved  in  the  absorptive  processes  are, 
primarily,  the  lymph-spaces,  the  lymph-capillaries,  and  the  blood-capillaries 
secondarily,  the  lymphatic  vessels  and  larger  blood-vessels. 

Lymph-spaces,  Lymph-capillaries,  Blood-capillaries. — Everywhere 
throughout  the  body,  in  the  intervals  between  connective-tissue  bundles 
and  in  the  interstices  of  the  several  structures  of  which  an  organ  is  com- 
posed, are  found  spaces  of  irregular  shape  and  size,  determined  largely  by 


ABSORPTION  87 

the  nature  of  the  organ  in  which  they  are  found,  which  have  been  termed 
lymph-spaces  or  lacuncB,  from  the  fact  that  during  the  living  condition 
they  are  continually  receiving  the  lymph  which  has  escaped  from  the  blood 
vessels  throughout  the  body.  In  addition  to  the  connective-tissue 
lymph-spaces,  various  observers  have  described  special  lymph-spaces  in 
the  testicle,  kidney,  liver,  thymus  gland,  and  spleen;  in  all  secreting  glands 
between  the  basement  membrane  and  blood-vessels;  around  blood-vessels 
(perivascular  spaces),  and  around  nerves.  The  serous  cavities  of  the  body 
— peritoneal,  pleural,  pericardial,  etc. — may  also  be  regarded  as  lymph- 
spaces,  which  are  in  direct  communication  by  open  mouths  or  stomata 
with  the  lymph  capillaries.  This  method  of  communication  is  not  only 
true  of  serous  membranes,  but  to  some  extent  also  of  mucous  membranes, 
The  cylindric  sheaths  and  endothelial  cells  surrounding  the  brain,  spinal 
cord,  and  nerves  can  also  be  looked  upon  as  lymph-spaces  in  connection 
with  lymph-capillaries. 

The  blood-capillaries  not  only  permit  the  passage  of  the  liquid  nutritive 
portions  of  the  blood  across  their  delicate  walls,  but  are  also  engaged  in 
the  reabsorption  of  this  transudate,  as  well  as  in  the  absorption  of  new 
materials  from  the  alimentary  canal.  The  extensive  capillary  network 
which  is  formed  by  the  ultimate  subdivision  of  the  arterioles  in  the  sub- 
mucous tissue  and  villi  of  the  small  intestine  forms  an  anatomic  arrange- 
ment well  adapted  for  absorption.  It  is  now  well  known  that  in  the 
obsorption  of  the  products**  of  digestion  the  blood-capillaries  are  more 
active  than  the  lymph-capillaries. 

The  Blood-vessels. — The  blood-vessels  which  are  concerned  in  the 
conduction  of  fresh  nutritive  material  from  the  alimentary  canal  have 
their  origin  in  the  elaborate  capillary  network  in  the  mucous  membrane. 
The  small  veins  which  emerge  from  the  network  gradually  unite,  forming 
larger  and  larger  trunks,  which  are  known  as  the  gastric,  superior,  and 
inferior  mesenteric  veins.  These  finally  unite  to  form  the  portal  vein,  a 
short  trunk  about  three  inches  in  length.  The  portal  vein  enters  the  liver 
at  the  transverse  fissure,  after  which  it  forms  a  fine  capillary  plexus 
ramifying  throughout  the  substance  of  the  liver;  from  this  plexus  the 
hepatic  veins  take  their  origin,  and  finally  empty  the  blood  into  the  vena 
cava  inferior.     (See  Fig.  9.) 

The  lymph-capillaries f  in  which  the  lymph-vessels  proper  take  their 
origin,  are  arranged  in  the  form  of  plexuses  of  quite  irregular  shape.  In 
most  situations  they  are  intimately  interwoven  with  the  blood-vessels, 
from  which,  however,  they  can  be  readily  distinguished  by  their  larger 
caliber  and  irregular  expansions.  The  wall  of  the  lymph-capillary  is 
formed  by  a  single  layer  of  epithelioid  cells,  with  sinuous  outlines,  and 


88  HUMAN   PHYSIOLOGY 

which  accurately  dove-tail  with  one  another.  In  no  instance  are  valves 
found.  In  the  villus  of  the  small  intestine  the  beginning  of  the  lymphatic 
is  to  be  regarded  as  a  lymph-capillary,  generally  club-shaped,  which  at 
the  base  of  the  villus  enters  a  true  lymphatic;  at  this  point  a  valve  is 
situated,  which  prevents  regurgitation.  The  lymph  capillaries  anasto- 
mose freely  with  one  another,  and  communicate  on  the  one  hand  with  the 
lymph-spaces,  and  on  the  other  with  the  lymphatic  vessels  proper. 


Fig.  9. — Diagram  of  the  portal  vein  {pv)  arising  in  the  alimentary  tract  and  spleen 
(s)  and  carrying  the  blood  from  these  organs  to  the  Hver. — {Yeo's  *' Text-book  oj 
Physiology.") 

As  the  shape,  size,  etc.,  of  both  lymph-spaces  and  capillaries  are  deter- 
mined largely  by  the  nature  of  the  tissues  in  which  they  are  contained,  it  is 
not  always  possible  to  separate  the  one  from  the  other.  Their  function, 
however,  may  be  regarded  as  similar — viz.,  the  collection  of  the  lymph 
which  has  escaped  from  the  blood-vessels,  and  its  transmission  onward 
into  the  regular  lymphatic  vessels. 

Lymph-vessels. — These  constitute  a  system  of  minute,  delicate  trans- 
parent vessels,  found  in  nearly  all  the  organs  and  tissues  of  the  body. 


\ 


ABSORPTION  89 

Having  their  origin  at  the  periphery  in  the  lymph-capillaries  and  spaces, 
they  rapidly  converge  toward  the  trunk  of  the  body  and  empty  into  the 
thoracic  duct.  In  their  course  they  pass  through  numerous  small  ovoid 
bodies,  the  lymphatic  glands. 

The  lymph- vessels  of  the  small  intestines — the  lacteals — arise  within  the 
villus  processes  which  project  from  the  inner  surface  of  the  intestine 
throughout  its  entire  extent.  The  wall  of  the  villus  is  formed  by  an 
elevation  of  the  basement  membrane,  and  is  covered  by  a  layer  of  columnar 
epithelial  cells.  The  basis  of  the  villus  consists  of  adenoid  tissue,  a  fine 
plexus  of  blood-vessels,  unstriped  muscle-fibers,  and  the  lacteal  vessel. 
The  adenoid  tissue  consists  of  a  number  of  intercommunicating  spaces, 
containing  leukocytes.  The  lacteal  vessel  possesses  a  thin  but  distinct 
.wall  composed  of  endothelial  plates,  with  here  and  there  openings  which 
bring  the  interior  of  the  villus  into  communication  with  the  spaces  of  the 
adenoid  tissue. 

The  structure  of  the  larger  vessels  resembles  that  of  the  veins,  consisting 
of  three  coats: 

1.  External,  composed  of  fibrous  tissue  and  muscle  fibers,  arranged 
longitudinally. 

2.  Middle,  consisting  of  white  fibers  and  yellow  elastic  tissue,  non- 
striated  muscle-fibers,  arranged  transversely. 

3.  Internal,  composed  of  an  elastic  membrane,  lined  by  endothelial  cells. 
Throughout  their  course  are  found  numerous  semilunar  valves,  opening 

toward  the  larger  vessels,  formed  by  a  folding  of  the  inner  coat  and 
strengthened  by  connective  tissue. 

Lymph  Glands. — The  lymph  glands  consist  of  an  external  capsule 
composed  of  fibrous  tissue  which  contains  non-striped  muscle-fibers;  from 
its  inner  surface  septa  of  fibrous  tissue  pass  inward  and  subdivide  the 
gland-substance  into  a  series  of  compartments,  which  communicate  with 
one  another.  The  blood-vessels  which  penetrate  the  gland  are  sur- 
rounded by  fine  threads,  forming  a  follicular  arrangement,  the  meshes  of 
which  contain  numerous  lymph-corpuscles.  Between  the  follicular 
threads  and  the  wall  of  the  gland  lies  a  lymph-channel  traviersed  by  a 
reticulum  of  adenoid  tissue.  The  lymph-vessels,  after  penetrating  this 
capsule,  pour  their  lymph  into  this  channel,  through  which  it  passes;  it  is 
then  collected  by  the  efferent  vessels  and  transmitted  onward.  The 
lymph-corpuscles  which  are  washed  out  of  the  gland  into  the  lymph- 
stream  are  formed,  most  probably,  by  division  of  preexisting  cells. 

The  Thoracic  Duct. — The  thoracic  duct  is  the  general  trunk  of  the 
lymphatic  system;  into  it  the  vessels  of  the  lower  extremities,  of  the 


90  HUMAN   PHYSIOLOGY 

abdominal  organs,  of  the  left  side  of  the  head,  and  of  the  left  arm  empty 
their  contents.  It  is  about  fifty  mm.  in  length,  arises  in  the  abdomen, 
opposite  the  third  lumbar  vertebra,  by  a  dilatation  (the  receptaculum  chyli), 
ascends  along  the  vertebral  column  to  the  seventh  cervical  vertebra,  and 
terminates  in  the  venous  system  at  the  junction  of  the  internal  jugular  and 
subclavian  veins  on  the  left  side.  The  lymphatics  of  the  right  side  of  the 
head,  of  the  right  arm,  and  of  the  right  side  of  the  thorax  terminate  in 
the  right  thoracic  duct,  about  one  inch  in  length,  which  joins  the  venous 
system  at  the  junction  of  the  internal  jugular  and  subclavian  on  the  right 
side. 
The  general  arrangement  of  the  lymph  vessels  is  shown  in  figure  lo. 

Lymph. — ^Lymph  is  a  clear  colorless  fluid  found  in  the  tissue  spaces  and 
in  the  lymph  vessels.  The  former  is  termed  intercellular,  the  latter  intra- 
vascular. 

Physical  Properties  and  Chemic  Composition. — ^Lymph  is  clear,  colorless, 
alkaline  in  reaction,  saline  to  the  taste  and  has  a  specific  gravity  varying 
from  I.020  to  1.030.  It  holds  in  suspension  a  number  of  corpuscles 
resembling  in  their  general  appearance  the  white  corpuscles  of  the  blood. 
Their  number  has  been  estimated  at  8,200  per  cubic  millimeter,  though  the 
number  varies  in  different  portions  of  the  lymphatic  system.  As  the 
lymph  flows  through  the  lymphatic  gland  it  receives  a  large  addition  of 
corpuscles.  Lymph-corpuscles  are  granular  in  structure,  and  measure 
3^,500  of  an  inch  in  diameter.  When  withdrawn  from  the  vessels,  lymph 
undergoes  a  spontaneous  coagulation  similar  to  that  of  blood,  after  which 
it  separates  in  serum  and  clot. 

Chemic  analysis  shows  that  it  consists  of  water,  proteins  3  to  4  per 
cent.,  fat  0.004  to  0.13  per  cent.,  sugar,  o.io  to  0.15  per  cent.,  inorganic 
salts,  0.8  to  0.9  per  cent.,  urea  CO2  and  other  katabolic  products  are 
present  in  small  amounts. 

Origin  and  Functions  of  Intercellular  Lymph. — Though  the  blood  is 
the  common  reservoir  of  all  nutritive  materials,  they  are  not  available 
for  nutritive  purposes  as  long  as  they  are  confined  within  the  blood- 
vessels. But  owing  to  the  character  of  the  wall  of  the  capillary  blood- 
vessels, some  of  the  constituents  of  the  blood-plasma  pass  across  it  and 
are  received  by  the  tissue-spaces  in  which  they  come  into  contact  with 
the  tissue-cells.  To  the  sum  total  of  these  materials  the  term  lymph 
is  given.  The  forces  concerned  in  the  passage  of  the  constituents  of  the 
lymph  across  the  capillary  wall  are  diffusion,  osmosis  and  filtration.  Other 
forces  have  been  suggested  such  as  the  secretory  activity  of  the  capillary 
cells  and  the  increased  concentration  of  the  lymph  due  to  the  accumula- 


ABSORPTION 


91 


Fig.  10. — Diagram  Showing  the  Course  of  the  Main  Trunk  of  the  Absorbent 
System. — (Yeo's  ''Textbook  of  Physiology,*') 
The  lymph-vessels  of  lower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC 
at  the  receptaculum  chyli  (RC),  where  the  thoracic  duct  begins.  The  superficial 
vessels  are  shown  in  the  diagram  on  the  right  arm  and  leg  (S) ,  and  the  deeper  ones 
on  the  arm  to  the  left  (D).  The  glands  are  here  and  there  shown  in  groups.  The 
small  right  duct  opens  into  the  veins  on  the  right  side.  The  thoracic  duct  opens 
into  the  union  of  the  great  veins  of  the  left  side  of  the  neck  (T). 


92  HUMAN   PHYSIOLOGY 

tion  of  katabolic  products  whereby  the  osmotic  pressure  is  increased. 
Its  function  becomes  apparent  from  its  origin  and  composition,  its  situa- 
tion and  relation  to  the  tissues.  It  is  to  furnish  the  tissue-cells  with  those 
nutritive  materials  which  are  necessary  for  their  growth,  repair  and  func- 
tional activity.  It  also  receives  all  waste  products  that  arise  from  their 
activity  prior  to  their  removal  by  the  blood-  and  lymph-vessels. 

Absorption  of  Intercellular  Lymph. — From  the  fact  that  lymph  is  being 
discharged  more  or  less  continuously  from  the  thoracic  duct,  it  is  evident 
that  lymph  is  being  absorbed  from  the  intercellular  spaces;  from  which  it 
may  be  inferred  that  more  lymph  is  passing  from  the  blood  into  the  tissue- 
space  than  is  necessary  for  the  immediate  needs  of  the  tissues.  To  pre- 
vent an  accumulation  and  an  interference  through  pressure,  with  the 
activities  of  the  tissues,  the  excess  is  absorbed  by  the  lymph-vessels  and 
returned  to  the  blood  stream  by  way  of  the  thoracic  duct.  It  is  likely 
that  some  of  the  constituents  are  also  absorbed  by  the  blood-vessels. 

Absorption  of  Food. — Physiological  experiments  have  demonstrated 
that  the  agents  concerned  in  the  absorption  of  new  materials  from  the 
alimentary  canal  are: 

1.  The  blood-vessels  of  the  entire  canal,  but  more  particularly  those 
uniting  to  form  the  portal  vein. 

2.  The  lymph  vessels  coming  from  the  small  intestine,  which  converge 
to  empty  into  the  thoracic  duct. 

As  a  result  of  the  action  of  the  digestive  fluids  upon  the  different  classes 
of  food  principles — proteins,  sugars,  starches,  and  fats — there  are  formed 
amino-acids,  dextrose  and  levalose,  soap  and  glycerin,  which  differ  from 
the  former  in  being  highly  diffusible — a  condition  essential  to  their 
absorption. 

Their  absorption  is  accomplished  by  the  villous  processes  covering  the 
surface  of  the  intestinal  mucous  membrane. 

The  Villi. — The  villi  are  small  filiform  or  conical  processes  projecting 
from  the  surface  of  the  mucous  membrane.  Each  villus  consists  of  a 
basement  membrane  supporting  columnar  epithelial  cells.  In  the  interior 
of  the  villus  there  is  frame  work  of  connective  tissue  supporting  arteries, 
capillaries  and  veins  and  a  single  club-shaped  lymph  capillary. 

Ftmction  of  the  Villi.^ — The  villi,  and  especially  the  epithelial  cells 
covering  them,  are  the  essential  agents  in  the  absorption  of  the  products 
of  digestion.  It  is  by  the  activity  of  these  cells  that  the  new  materials 
are  taken  out  of  the  alimentary  canal  and  transferred  into  the  lymph- 
spaces  in  the  interior  of  the  villi,  from  which  they  are  subsequently 
removed  by  the  blood-vessels  and  lymph- vessels. 


^. 


ABSORPTION  93 

The  water  and  inorganic  salts  and  sugars  after  their  absorption  by  the 
epithelium  of  the  villi  pass  onward  into  the  interior  of  the  villi;  thence 
across  the  capillary  wall  into  the  blood  by  which  they  are  carried  to  the 
liver.  The  water  and  salts  in  all  probability  pass  directly  through  the 
iver  to  become  part  of  the  general  blood  volume.  The  sugar  is  in  part 
removed  from  the  blood  stream  and  temporarily  stored  in  the  liver  cells 
under  the  form  of  starch.  As  it  subsequently  is  transformed  into  glycose  or 
glucose  it  was  termed  glycogen.  The  products  of  protein  digestion — the 
amino-acids — are  also  absorbed  by  the  epithelial  cells.  It  was  until 
recently  believed  that  during  their  transit  through  the  cell  they  were  syn- 
thesized into  plasma-albumin  which  was  then  discharged  into  the  blood 
stream.  More  recent  experiments  indicate  that  this  is  not  the  case  and  that 
the  amino-acids  pass  directly  into  and  are  found  circulating  in  the  blood. 

The  products  of  fat  digestion — soap  and  glycerin — after  absorption 
are  synthesized  to  fat  which  is  deposited  in  the  epithelial  cells  in  the  form 
of  small  drops,  after  which  it  too  passes  to  the  interior  of  the  villus  to 
enter  the  lymph  capillary. 

The  products  oj  digestion  find  their  way  into  the  general  circulation  by 
two  routes: 

1.  The  water,  protein,  dextrose,  and  soluble  salts,  after  passing  into  the 
lymph-spaces  of  the  villi,  pass  across  the  wall  of  the  capillary  blood- 
vessel; entering  the  blood,  they  are  carried  to  the  liver  by  the  vessels 
uniting  to  form  the  portal  vein.  Fig.  9;  emerging  from  the  liver,  they  are 
emptied  into  the  inferior  vena  cava  by  the  hepatic  vein. 

2.  The  fat  enters  the  lymph-capillary  in  the  interior  of  the  villus;  by  the 
contraction  of  the  layer  of  muscle-fibers  surrounding  it,  its  contents  are 
forced  onward  into  the  lymph-vessels,  thence  into  the  thoracic  duct,  and 
finally  into  the  blood  stream  at  the  junction  of  the  internal  jugular  and 
subclavian  veins  on  the  left  side.  Fig.  10. 

Chyle. — Chyle  is  the  fluid  found  in  the  lymph  vessels,  coming  from  the 
small  intestine  after  the  digestion  of  a  meal  containing  fat.  In  the 
intervals  of  digestion  the  fluid  of  these  lymphatics  is  identical  in  all  respect 
with  the  lymph  found  in  all  other  regions  of  the  body.  As  soon  as  the 
granular  fat  passes  into  the  lymph  vessels  and  mingles  with  the  lymph 
it  becomes  milky  white  in  color,  and  the  vessels  which  previously  were  in- 
visible become  visible,  and  resemble  white  threads  running  between  the 
layers  of  the  mesentery.  Chyle  has  a  composition  similar  to  that  of 
lymph,  but  it  contains,  in  addition,  numerous  fatty  granules.  When 
examined  microscopically,  the  chyle  presents  a  fine  molecular  basis,  made 
up  of  the  finely  divided  granules  of  fat. 


94  HUMAN  PHYSIOLOGY 

Forces  Aiding  the  Movement  of  Lymph  and  Chyle. — The  lymph  and 
chyle  are  continually  moving  in  a  progressive  manner  from  the  periphery 
or  beginning  of  the  lymphatic  system  to  the  final  termination  of  the 
thoracic  duct.  The  force  which  primarily  determines  the  movement  of 
the  lymph  has  its  origin  in  the  beginnings  of  the  lymph-vessels,  and 
depends  upon  the  difference  in  pressure  here  and  the  pressure  in  the 
thoracic  duct.  The  greater  the  quantity  of  fluid  poured  into  the  lymph- 
spaces,  the  greater  will  be  the  pressure  and,  consequently,  the  move- 
ment. The  first  movement  of  chyle  is  the  result  of  a  contraction  of  the 
muscle-fibers  within  the  walls  of  the  villus.  At  the  time  of  coiitraction 
the  lymph  capillary  is  compressed  and  shortened,  and  its  contents  are 
forced  onward  into  the  true  lymphatic.  When  the  muscle-fibers  relax, 
regurgitation  is  prevented  by  the  closure  of  the  valve  in  the  lymphatic  at 
the  base  of  the  villus. 

As  the  walls  of  the  lymph  vessels  contain  muscle-fibers,  when  they 
become  distended  these  fibers  contract  and  assist  materially  in  the  onward 
movement  of  the  fluid. 

The  contraction  of  the  general  muscular  masses  in  all  parts  of  the  body,  ' 
by  exerting  an  intermittent  pressure  upon  the  lymphatics,  also  hastens 
the  current  onward;  regurgitation  is  prevented  by  the  closure  of  valves 
which  everywhere  line  the  interior  of  the  vessels. 

The  respiratory  movements  aid  the  general  flow  of  both  lymph  and  chyle 
from  the  thoracic  duct  into  the  venous  blood.  During  the  time  of  an 
inspiratory  movement  the  pressure  within  the  thorax,  but  outside  the 
lungs,  undergoes"  a  diminution  in  proportion  to  the  extent  of  the  move- 
ment; as  a  result,  the  fluid  in  the  thoracic  duct  outside  of  the  thorax, 
being  under  a  higher  pressure,  flows  more  rapidly  into  the  venous  system. 
At  the  time  of  an  expiration,  the  pressure  rises  and  the  flow  is  temporarily 
impeded,  only  to  begin  again  at  the  next  inspiration. 

THE  BLOOD 

The  blood  may  be  defined  as  the  nutritive  fluid  of  the  body  since  it 
contains  all  those  materials  that  are  necessary  to  the  maintenance  of  the 
nutrition.  The  presence  and  proper  circulation  of  the  blood  in  the  living 
organism  are  essential  for  the  maintenance  of  tissue  irritability  and  for 
the  manifestation  of  the  activities  of  all  physiologic  mechanisms.  The 
escape  of  the  blood  from  the  vessels,  especially  in  the  higher  animals,  is 
followed  by  cessation  of  the  physiologic  activities  of  all  the  tissues  within  a 
short  period.  The  irritability,  however,  persists  for  a  variable  length  of 
time  though  it  too  gradually  declines  and  finally  disappears.    The  blood 


THE  BLOOD  .  95 

is  also  a  reservoir  for  the  reception  of  katabolic  products  produced  by 
and  absorbed  from  the  tissues. 

The  Physical  Constitution  of  Blood. — A  microscopic  examination  of 
the  blood  as  it  flows  through  the  capillary  vessels  of  the  web  of  the  frog  or 
the  mesentery  of  the  rabbit  shows  that  it  is  not  a  homogenous  fluid,  but 
that  it  consists  of  two  distinct  portions,  viz.:  (i)  a  clear,  transparent, 
slightly  yellow  fluid,  the  plasma  or  liquor  sanguinis:  (2)  small  particles 
termed  corpuscles  floating  in  it,  of  which  there  are  two  varieties,  the  red 
or  the  erythrocytes  and  the  white  or  the  leukocytes.  By  appropriate 
thods  it  can  be  shown  that  a  third  corpuscle,  colorless  in  appearance 
smaller  in  size  than  the  ordinary  white  corpuscle,  is  presg^M  the  blood 
stream  and  known  as  the  blood-platelet  or  ph 


Physical  Properties. — The  color  of  the  blooHWpaarteries  is  scarlet  red, 
in  the  veins  bluish  red.  The  cause  of  the  color  is  the  presence  of  a  col- 
oring matter,  hemoglobin,  in  different  degrees  of  combination  with  oxygen . 
As  the  venous  blood  passes  into  and  through  the  pulmonic  capillaries  the 
hemoglobin  absorbs  a  certain  volume  of  oxygen  after  which  it  changes 
in  color  and  on  emerging  from  the  lungs  imparts  to  the  blood  its  charac- 
teristic scarlet-red  color.  By  reason  of  the  union  of  the  hemoglobin  with 
the  oxyg/en  it  is  generally  termed  while  in  the  arteries,  oxyhemoglobin. 
As  the  arterial  blood  passes  into  and  through  the  systemic  capillaries,  the 
oxyhemoglobin  yields  up  a  portion  of  its  oxygen  to  the  tissues  after 
'which  it  again  changes  in  color  and  on  emerging  from  the  tissues  imparts 
to  the  blood  its  characteristic  bluish-red  color.  By  reason  of  the  loss  of  a 
portion  of  its  oxygen,  the  hemoglobin  is  generally  termed  while  in  the 
veins,  deoxy-  or  reduced  hemoglobin. 

The  opacity  of  the  blood  or  the  inability  to  see  objects  through  it,  is  the 
result  of  the  dissipation  of  light,  caused  by  the  shape  of  the  red  corpuscles. 

The  specific  gravity  within  the  limits  of  health,  ranges  from  1.045  to 
1.075  though  the  average  is  about  1.056. 

The  reaction  of  the  blood  is  stated  to  be  slightly  alkaline  because  of  the 
fact,  as  determined  by  the  methods  of  physical  chemistry,  there  is  a 
preponderance  of  the  hydroxyl  ions  over  the  hydrogen  ions.  The  alkaline 
bases  introduced  into  the  blood  in  the  foods  are  always  in  excess  of  the 
acids  produced  under  physiological  conditions  and  hence  they  constitute 
an  "alkaline  reserve."  The  physiologic  action  of  this  reserve  is  to  com- 
bine with  and  remove  from  the  body  the  acids,  e.g.,  sulphuric  and  phos- 
phoric acids  that  arise  during  the  metabolism  of  the  proteins. 

The  temperature  varies  in  different  regions.  In  the  aorta  it  is  approxi- 
mated 38.6°C.;  in  the  portal  vein  39°C.;  in  the  hepatic  veins  39.7°C. 


96  HUMAN   PHYSIOLOGY 

The  viscosity  or  the  resistance  to  the  movement  of  the  molecules  of 
the  plasma  among  themselves,  together  with  that  of  the  corpuscles,  is 
considerable.  Compared  with  distilled  water  the  viscosity  of  human 
blood  is  4.5'  times  as  great.  The  viscosity  is  increased  and  decreased  by  a 
rise  or  fall  in  the  number  of  red  corpuscles.  In  a  case  of  polycythemia, 
the  red  corpuscle  count  was  1 1 ,000,000  per  cubic  millimeter  and  the  vis- 
cosity 3  and  4  times  the  normal. 

Coagulability. — When  blood  is  withdrawn  from  the  body  and  allowed 

to  remain  at  rest,  it  becomes  somewhat  thick  and  viscid  in  from  three  to 

ve  minutes;  this  viscidity  gradually  increases  until  the  entire  volume  of 

blood  assumes  a  jelly-like  consistence,  which  process  occupies  from  five 

to  fifteen  minutes. 

If  a  portion  of  such  a  jelly-like  mass  be  examined  microscopically,  it 
will  be  found  to  be  penetrated  in  all  directions  by  a  felt-work  of  extremely 
fine  delicate  fibrils,  which,  having  made  their  appearance  before  the 
corpuscles  have  had  time  to  settle  to  the  bottom  of  the  fluid,  have  en- 
tangled them  in  the  meshes  so  that  the  entire  mass  retains  its  characteristic 
color.  These  fibrils  are  collectively  known  as  fibrin j,^|ffhe  appearance  of 
the  fibrin  is,  therefore,  the  cause  of  the  coagulation /^|^^ 

As  soon  as  coagulation  is  completed,  a  second  proems  begins,  which 
consists  in  the  contraction  of  the  coagulum  and  the  oozing  of  a  clear, 
straw-colored  liquid — the  serum — which  gradually  increases  in  quantity 
as  the  clot  diminishes  in  size,  by  contraction,  until  the  separation  ij 
completed,  which  occupies  from  twelve  to  twenty-four  hours. 


The  Cause  of  Coagulation. — Coagulation  is  due  to  the  appearance 
fibrin,  a  compound  formed  by  a  physico-chemic  union  of  an  organic 
colloidal  body,  thrombin^  with  fibrinogen,  this  latter  substance  being 
always  present  in  the  blood.  Thrombin  is  believed  to  be  a  derivative 
of  an  antecedent  substance  prothrombin  or  thrombogen,  a  substance 
always  present  in  the  blood  and  is  a  product  of  the  decomposition  of 
leukocytes  and  the  blood-platelets.  With  thrombin  there  is  associated  a 
calcium  salt  which  is  essential  for  coagulation.  If  it  is  removed  by  the 
addition  of  potassium  oxalate  coagulation  does  not  take  place.  These 
three  substances  prothrombin  or  thrombogen,  a  calcium  salt  and  fibrino- 
gen are  always  present  in  the  blood.  The  formation  of  thrombin  which 
would  cause  coagulation  is  prevented  by  the  presence  of  an  an ti- thrombin. 
As  soon  as  blood  is  shed  or  tissue  are  injured  a  new  substance  throm- 
binoplastin  is  developed  which  neutralizes  the  anti-thrombin.  This  hav 
ing  been  accomplished  the  calcium  is  enabled  to  activate  the  prothrombin 
with  the  production  of  thrombin  and  hence  fibrin  (Howell). 


THE  BLOOD  97 

Conditions  Influencing  Coagulation. — The  process  is  retarded  by  cold, 
retention  within  living  normal  vessels,  neutral  salts  in  excess,  the  injection 
of  commercial  peptone,  etc. 

It  is  accelerated  by  a  temperature  of  i.oo°F.,  contact  with  rough  surfaces, 
the  presence  of  foreign  bodies,  whipping,  etc. 

Blood  coagulates  in  the  body  after  the  arrest  of  the  circulation  in  the 
course  of  twelve  to  twenty-four  hours;  local  arrest  of  the  circulation,  from 
compression  or  a  ligature  with  injury  to  the  lining  membrane  of  the  vessel, 
will  cause  coagulation,  thus  preventing  hemorrhages  from  wounded 
vessels. 

Chemic  Composition  of  Plasma. — An  average  composition  of  plasma 
is  shown  in  the  following  table: 

Water 90 .  oo 

i  Plasma-albumin 4-50 

Paraglobulin 3  •  40 

Fibrinogen 0.30 

Fatty  matters 0.25 

Sugar o .  10 

Extractives 0 .  60 

Inorganic  salts    0.85 


The  water  imparts  fluidity  to  the  blood  and  acts  as  a  solvent  for  the 
inorganic  matters,  for  sugar,  and  various  products  of  katabolism. 

Plasma-albumin  was  formerly  regarded  as  the  nutritive  protein  of  the 
blood  and  directly  used  as  such  by  the  tissue  elements.  As  the  amino- 
acids  are  now  believed  to  play  this  r6le,  the  albumin  must  have  some  other 
function.    It  may  be  a  reserve  supply  of  protein  food. 

Paraglobulin  is  a  soft,  amorphous  substance  precipitated  by  sodium 
chlorid  in  excess,  or  by  passing  a  stream  of  carbonic  acid  through  dilute 
serum. 

Fibrinogen  also  can  be  obtained  by  strongly  diluting  the  serum  and  pass- 
ing carbonic  acid  through  it  for  a  long  time,  when  it  is  precipitated  as  a 
viscous  deposit. 

Fatly  matter  exists  in  the  blood  to  the  extent  of  about  0.25  per  cent.  Just 
after  a  meal  rich  in  fat,  this  amount  may  be  considerably  increased. 
Within  a  few  hours  it  disappears,  though  its  ultimate  fate  is  unknown. 

Sugar  is  represented  by  dextrose.  The  amount  present  varies  from  0.8 
to  0.15  per  cent.  It  is  derived  directly  from  the  glycogen  of  the  liver. 
Should  the  normal  percentage  be  increased,  the  excess  will  be  eliminated 
by  the  kidneys  and  the  condition  of  glycosuria  is  established. 

The  inorganic  constituents  are  chiefly  sodium  and  potassium  chlorids, 
7 


98  HUMAN  PHYSIOLOGY 

sulphates  and  phosphates  together  with  calcium  and  magnesium  phos- 
phates. The  sodium  chlorid  is  the  most  abundant,  amounting  to  about 
5-5  parts  per  thousand.  The  alkaline  salts  impart  the  alkaline  reaction 
and  promote  the  absorption  from  the  tissues  of  the  carbon  dioxid. 

Excrementitious  matters  are  represented  by  carbonic  acid,  urea,  creatin, 
creatinin,  urates,  oxalates,  etc.;  they  are  absorbed  from  the  tissues  by  the 
blood  and  conveyed  to  the  excretory  organs,  lungs,  kidneys,  etc. 

Gases. — Oxygen,  nitrogen,  and  carbonic  acid  exist  in  varying  propor- 
tions. 

The  serum  differs  from  plasma  in  not  containing  those  materials  which 
entered  into  the  formation  of  fibrin. 

THE  RED  CORPUSCLES  OR  ERYTHROCYTES 

The  red  corpuscles  are  circular  biconcave  flattened  discs  having  an 
average  diameter  of  0.007  i^ni-  or  about  M,200  of  an  inch.  A  single  cor- 
puscle is  of  a  pale  straw  color.  It  is  only  when  aggregated  in  masses 
that  they  assume  a  red  color.  In  man  and  mammals  the  red  corpuscles 
present  neither  a  nucleus  nor  a  cell  wall  and  are  universally  of  a  small  size, 
though  the  size  varies  considerable  in  different  mammals. 

The  red  corpuscles  are  exceedingly  numerous,  amounting  to  about 
5,000,000  in  a  cubic  millimeter  of  blood.  In  structure  they  consist  of  a 
firm,  elastic,  colorless  framework — the  stroma — in  the  meshes  of  which  is 
entangled  the  coloring-matter — the  hemoglobin. 

According  to  some  histologists  the  red  corpuscle,  while  in  the  plasma, 
assumes  a  bell  shape.  The  circular  biconcave  shape  usually  observed 
under  the  microscope  is  regarded  as  due  to  cooling  and  evaporation  and 
concentration  of  the  drawn  blood. 

In  the  birds,  reptiles,  amphibia,  and  fish  the  red  corpuscles  are  oval  in 
shape  and  have  a  distinct  nucleus.  They  can,  therefore,  be  readily  dis- 
tinguished from  the  corpuscles  of  mammals,  not  only  by  their  structure 
but  also  by  their  size,  which  is  distinctly  larger. 

Chemic  Composition  of  Red  Corpuscles. — When  analyzed  chemically 
the  red  corpuscles  are  found  to  consist  of  water  65  per  cent,  and  solid 
matter  35  per  cent.  The  solids,  moreover,  have  been  found  to  consist  of 
a  pigment  hemoglobin  7,3^  protein  0.9,  cholesterin  and  lecithin  0.46,  and 
inorganic  salts  (chiefly  potassium  phosphate  and  chlorid  and  sodium 
chlorid)  1.4  per  cent,  respectively.  Of  the  total  solids  the  hemoglobin 
constitutes  about  94  per  cent. 

Hemoglobin,  the  coloring-matter  of  the  corpuscle,  is  an  albuminous 
compound,  composed  of  C,0,H,N,S,  and  iron.    It  may  exist  in  either 


THE  BLOOD  99 

an  amorphous  or  a  crystalline  form.  When  deprived  of  all  its  oxygen, 
except  the  quantity  entering  into  its  intimate  composition,  the  hemoglobin 
becomes  purplish  in  color,  and  is  known  as  reduced  hemoglobin.  When  ex- 
posed to  the  action  of  oxygen,  it  again  absorbs  a  definite  amount  and  be- 
comes scarlet  in  color,  and  is  known  as  oxyhemoglobin.  The  amount  of 
oxygen  absorbed  is  1.34  c.c.  for  each  gram  of  hemoglobin. 

It  is  this  substance  which  gives  the  color  to  the  venous  and  arterial 
blood.  As  the  venous  blood  passes  through  the  capillaries  of  the  lungs 
the  reduced  hemoglobin  absorbs  the  oxygen  from  the  pulmonary  air  and 
becomes  oxyhemoglobin,  scarlet  in  color;  the  blood  becomes  arterial. 
When  the  arterial  blood  passes  into  the  systemic  capillaries,  the  oxygen 
is  absorbed  by  the  tissues;  the  hemoglobin  becomes  reduced,  purple  in 
color,  and  the  blood  becomes  venous.  A  dilute  solution  of  oxyhemoglobin 
gives  two  absorption  bands  between  the  lines  D  and  E  of  the  solar  spec- 
trum. Reduced  hemoglobin  gives  but  one  absorption  band,  occupying 
the  space  existing  between  the  two  bands  of  the  oxyhemoglobin  spectrum. 

The  Fimction  of  the  Red  Corpuscles. — The  red  corpuscles,  by  virtue 
of  the  capacity  of  their  contained  hemoglobin  for  oxygen  absorption,  may 
be  regarded  as  carriers  of  oxygen  from  the  lungs  to  the  tissues,  and  there- 
fore important  factors  in  the  general  respiratory  process.  The  size  as  well 
as  the  number  of  the  corpuscles  in  different  classes  of  animals  appears  to 
be  directly  related  to  the  activity  of  the  respiratory  process.  In  those 
animals  in  which  the  corpuscles  are  small  and  numerous  and  the  total 
superficial  area  large,  respiration  is  active,  the  quantity  of  oxygen  absorbed 
is  large,  and  the  energy  liberated  through  oxidation  is  correspondingly 
large.  In  those  animals,  on  the  contrary,  in  which  the  corpuscles  are 
large  and  relatively  few  in  number,  the  reverse  conditions  obtain.  This 
is  in  accordance  with  the  fact  that  the  superficial  area  of  any  given  volume 
of  substance  is  increased  in  proportion  to  the  extent  to  which  it  is  sub- 
divided. 

Origin. — The  red  corpoiscles  are  derived  from  erythroblasts  found  in 
the  red  marrow  of  the  long  bones.  In  the  passages  of  the  capillary  net- 
work of  the  marrow,  the  erythroblasts  make  their  appearance. jnos<t. 
probably  by  a  transformation  of  pre-existing  man  aw.  cells  which  cress  the 
capillary  wall  from  without.  At  first  they  ar»?  l<?r^e,:l:a)mdgeneous,  color- 
less, perhaps  slightly  tinged  with  hemoglobin  and  distinctly  nucleated: 
They  increase  in  number  by  karyokinesis  arxJ^at  th^i  samie  time  increasein* 
their  hemoglobin  content.  In  the  course  of  their  development  the  nucleus 
becomes  smaller  and  denser,  when  the  cells  are  known  as  normoblasts. 
Subsequently  the  nucleus  is  extruded,  carrying  with  it  a  portion  of  the 


100  HUMAN  PHYSIOLOGY 

perinuclear  cytoplasm,  after  which  the  remainder  of  the  corpuscle  assumes 
the  shape  and  size  of  the  adult  corpuscle  and  is  carried  out  into  the  general 
circulation.  After  severe  hemorrahge  the  formative  processes  in  the 
marrow  may  become  so  active  that  erythroblasts  and  normoblasts  make 
their  appearance  in  the  blood-stream  before  the  extrusion  of  the  nucleus 
has  taken  place. 

THE  WHITE  CORPUSCLES  OR  LEUKOCYTES 

The  white  corpuscle  is  grayish  in  color,  round  or  globular  in  form 
though  often  presenting  a  more  or  less  irregular  surface.  Its  diameter  is 
about  o.oii  mm.  or  about  J^,5oo  oi  an  inch.  Some  of  the  white  cor- 
puscles are,  however,  somewhat  larger  and  others  smaller. 

A  typical  white  corpuscle  consists  of  a  ground  substance  uniformly 
transparent  and  apparently  homogeneous  in  which  are  embedded  a  num- 
ber of  granules  of  varying  size,  some  of  which  are  very  fine,  while  others 
are  large.  By  various  reagents  it  has  been  demonstrated  that  the  gran- 
ules are  fatty,  protein,  and  carbohydrate  (glycogen)  in  character.  In 
the  fresh  cells  the  existence  of  a  nucleus  is  difiicult  of  detection,  though  its 
presence  can  be  demonstrated  by  the  addition  of  acetic  acid,  which  renders 
the 'perinuclear  cytoplasm  more  transparent  and  makes  the  nucleus  con- 
spicuous and  sharply  defined. 

The  number  of  white  corpuscles  per  cubic  millimeter  of  blood  is  much 
less  than  the  number  of  red  corpuscles,  the  ratio  being  in  the  neighborhood 
of  I  white  to  700  red.  This  ratio,  however,  varies  within  wide  limits  in 
different  portions  of  the  body  and  under  normal  variations  in  physiologic 
conditions.  In  the  blood  of  the  splenic  artery  there  is  but  i  white  to  2,260 
red,  while  in  the  splenic  vein  there  is  i  white  to  every  60  red;  or  about 
thirty-eight  times  as  many  as  in  the  artery.  In  the  portal  vein  there  is  i 
white  to  740  red,  while  in  the  hepatic  vein  there  is  i  white  to  1 70  red. 

The  total  number  of  white .  corpuscles  per  cubic  millimeter  has  been 
estimated  at  from  5,000  to  10,000,  though  the  average  is  about  7,500. 
The  number,  however,  is  influenced  by  a  variety  of  physiologic  conditions. 

The  white  corpuscles  are  usually  divided  into 

V     %       .     /  Sn;iall  25  percent. 

,1.  Lymp^ocrtes  .  ^^^    :  ^  ^^    g  p^^  ^^^^ 

.        ^     ,  '       J' Polymorphonuclear  60  to  70  per  cent. 

., .'Leukocytes.  .|  ^^^,^^^,  0.5  to    2  per  cent. 

In  abnormal  condition*  of  ith^  blood  other  forms  of  leukocytes  are  fre- 
quently present,  e.g.,  myelocytes,  leukoblasts,  myeloplaxes,  etc.,  the 
significance  of  which  is  not  always  apparent. 


THE  BLOOD  Iq^ 

Properties  -The  white  corpuscles  possess  the  power  of  spontaneous 
movement  alternately  contracting  and  expanding,  throwing  out  processes 
of  their  substance  and  quickly  withdrawing  them,  thus  changing  their 
shape  from  moment  to  moment.  These  movements  resemble  those  of 
the  ameba,  and  for  this  reason  are  termed  ameboid.  The  white  corpuscles 
also  possess  the  capability  of  passing  through  the  walls  of  the  capillaries 
into  the  surrounding  tissue  spaces;  to  this  process  the  term  diapedesis  is 
given. 

Functions.-The  functions  of  the  white  corpuscles  are  but  imperfectly 
known,  and  at  present  no  positive  statements  can  be  made.  It  has  been 
suggested  that  wherever  found  in  the  body,  whether  in  blood  or  tissues, 
they  are  engaged  in  the  removal  of  more  or  less  insoluble  particles  of  dis- 
integrated tissues,  in  attacking  and  destroying  more  or  less  effectively 
various  forms  of  invading  bacteria  and  thus  protecting  the  body  against 
their  deleterious  activity.  This  they  do  by  surrounding,  enveloping,  and 
incorporating  either  the  tissue  particle  or  bacterium  and  digesting  it 
Un  account  of  this  swallowing  action  these  ceUs  were  termed  by  Metchni- 
koff  phagocytes  and  the  process  phagocytosis.  The  cells  engaged  in  this 
process  are  the  polymorphonuclear  leukocytes  and  the  large  and  the  small 
lymphocytes.  He  regards  them  as  the  general  scavengers  of  the  body. 
It  has  been  suggested  that  they  are  also  engaged  in  the  absorption  of  fat 
rom  the  lymphoid  tissue  of  the  intestine.  In  their  dissolution  they  con- 
tribute  to  the  blood-plasma  certain  protein  materials  which  assist  under 
tavorable  circumstances  in  the  coagulation  of  the  blood. 

Origin.— The  first  group  of  the  white  corpuscles— lymphocytes— take 
their  origin  entirely  from  the  lymph-adenoid  tissues  of  the  body,  e.g.,  the 
lymph-glands,  solitary  and  agminated  follicles  of  the  intestines,  etc.  As 
the  lymph  flows  through  these  structures  the  lymph-corpuscles,  as  the 
future  lymphocytes  of  the  blood  are  caUed  in  these  situations,  are  washed 
out  and  carried  by  way  of  the  lymph-stream  into  the  general  circulation. 

The  second  group— the  polymorphonuclear,  the  eosinophiles  and  baso- 
phile  leukocytes  have  their  origin  in  the  bone  marrow.  The  immediate 
ancestors  of  these  cells  are  known  as  myelocytes  and  are  normally  found 
m  the  red  bone-marrow.  These  ceUs,  through  transitional  stages,  assume 
the  characteristics  of  the  leukocytes  just  mentioned  and  pass  directly 
into  the  capillaries  of  the  marrow  whence  they  are  distributed  throughout 
the  body. 

After  an  unknown  period  of  life  the  leukocytes  undergo  dissolution  and 
disappear. 


102  HUMAN  PHYSIOLOGY 

Blood  Platelets. — These  are  small  histologic  elements  circulating  in 
the  blood  though  their  presence  can  not  be  readily  determined  except 
under  special  conditions.  They  are  colorless  homogeneous  or  finely 
granular  non-nucleated  disks  which  vary  in  diameter  from  1.5  to  35 
micro-millimeters.  They  number  from  250,000  to  300,000  per  cubic 
millimeter  of  blood.  They  are  supposed  to  represent  fragments  of  the 
cystoplasm  of  giant  cells  found  in  the  marrow  of  bones.  They  are  be- 
lieved to  be  connected  in  some  way  with  the  coagulation  of  the  blood. 

THE  CIRCULATION  OF  THE  BLOOD 

The  circulatory  apparatus  by  which  the  blood  is  distributed  to  and  re- 
moved from  all  regions  of  the  body  consists  of  a  central  organ,  the  heart; 
a  series  of  branching  diverging  tubes,  the  arteries;  a  network  of  minute 
passageways  with  extremely  delicate  walls,  the  capillaries;  a  series  of  con- 
verging tubes,  the  veins.  These  structures  are  so  arranged  as  to  form  a 
closed  system  of  vessels  within  which  the  blood  is  kept  in  continuous  move- 
ment mainly  by  the  pressure  produced  by  the  pumping  action  of  the  heart, 
though  aided  by  other  forces.  By  reason  of  its  general  arrangement 
and  activity  the  tissues  are  continuously  supplied  with  nutritive  materials 
and  freed  from  their  waste  materials  and  carried  to  the  eliminating  organs. 

The  Heart. — The  heart  is  a  conic  or  pyramid-shaped  hollow  muscle 
situated  in  the  thorax  just  behind  the  sternum.  The  base  is  directed  up- 
ward and  to  the  right  side;  the  apex  downward  and  to  the  left  side,  extend- 
ing as  far  as  the  space  between  the  cartilages  of  the  fifth  and  sixth  ribs. 
In  this  situation  the  heart  is  enclosed  and  suspended  in  a  fibro-serous  sac, 
the  pericardium^  attached  to  the  great  vessels  at  its  base. 

Cavities  of  the  Heart. — The  general  cavity  of  the  heart  is  subdivided 
by  a  longitudinal  septum  into  a  right  and  left  half;  each  of  these  cavities  is 
in  turn  subdivided  by  a  transverse  septum  into  two  smaller  cavities,  which 
communicate  with  each  other  and  are  known  as  the  auricles  and  ventricles; 
the  orifice  between  the.  auricle  and  ventricle  being  known  as  the  auricula- 
ventricukv.  orifice.  The  heart , therefore,  consists  of  four  cavities — a  right 
auricle  and  ventricle  and  a  left  auricle  and  ventricle. 

The  right  auricle  and  the  right  ventricle  constitute  the  venous  heart; 
the  left  auricle  and  left  ventricle  constitute  the  arterial  heart. 

Into  the  right  auricle  the  two  terminal  trunks  of  the  venous  system — the 
superior  and  inferior  vena  cava — empty  the  venous  blood  which  has  been 
collected  from  all  parts  of  the  system;  from  the  right  ventricle  arises  the 
pulmonic  artery,  which,  passing  into  the  lungs,  distributes  the  blood  to  the 


CIRCULATION   OF  THE  BLOOD 


103 


walls  of  the  air-cells  of  the  lungs;  into 
the  left  auricle  empty  four  pulmonic 
veins,  which  have  collected  the  blood 
from  the  lung  capillaries;  from  the  left 
ventricle  springs  the  aorta,  the  general 
trunk  of  the  arterial  system,  the  branches 
of  which  distribute  the  blood  to  the 
entire  system. 

The  Course  of  the  Blood  through  the 
Heart. — Reference  to  Fig.  1 1  will  make 
it  clear  that  there  is  a  pathway  for  the 
blood  between  the  venae  cavae  on  the 
right  side  and  the  aorta  on  the  left  side 
by  way  of  the  right  side  of  the  heart, 
the  cardio-pulmonic  vessels  and  the  left 
side  of  the  heart. 

The  venous  blood  flowing  toward  the 
heart  is  emptied  by  the  superior  and 
inferior  vena  cava  into  the  right  auricle 
from  which  it  passes  through  the  auric- 
uloventricular  opening  into  the  right 
ventricle;  thence  into  and  through  the 
pulmonic  artery  and  its  branches  to  the 
pulmonic  capillaries  where  it  is  arterial- 
ized,  i.e.,  yields  up  a  portion  of  its  car- 
bon dioxid  and  takes  on  a  fresh  supply 
of  oxygen — and  is  changed  in  color  from 
bluish  red  to  scarlet  red.  The  arterial- 
ized  blood  flowing  toward  the  heart  is 
emptied  by  the  pulmonic  veins  into  the 
left  auricle  from  which  it  passes  through 
the  auriculoventricular  opening  into  the 
left  ventricle;  thence  into  the  aorta  and 
its  branches  to  the  systemic  capillaries 
where  it  is  dearterialized  by  an  oppo- 
site exchange  of  gases,  i.e.,  yields  up  a 
portion  of  its  oxygen  to,  and  absorbs 

Fig.  II. — Diagram  of  the  Circulation. 

I  I.  Heart.  2.  Lungs.  3.  Head  and 
upper  extremities.  4.  Spleen.  5.  Intestines. 
6.  Kidney.  7.  Lower  extremities.  8.  Liver. 
—{After  Dalton.) 


I04  HUMAN   PHYSIOLOGY 

carbon  dioxid  from  the  tissues,  and  changes  in  color,  from  scarlet  red  to 
bluish  red.  The  venous  blood  is  again  returned  to  the  systemic  veins  to 
the  venae  cavae. 

Though  the  blood  is  thus  described  as  flowing  first  through  the  right 
side  and  then  through  the  left  side,  it  must  be  kept  in  mind  that  the  two 
sides  fill  synchronously;  that  while  the  blood  is  flowing  into  the  right  side 
from  the  venae  cavae,  it  is  also  flowing  into  the  left  side  from  the  pulmonic 
veins  in  equal  quantities  and  velocities. 

While  there  is  but  one  circulation,  physiologists  frequently  divide  the 
circulatory  apparatus  into: 

1.  The  systemic  circulation,  which  includes  the  movement  of  the  blood 
from  the  left  side  of  the  heart  through  the  aorta  and  its  branches,  through 
the  capillaries  and  veins,  to  the  right  side  of  the  heart. 

2.  The  pulmonic  circulation,  which  includes  the  course  of  the  blood 
from  the  right  side  through  the  pulmonic  artery,  through  the  capillaries 
of  the  lungs  and  pulmonic  veins,  to  the  left  side  of  the  heart. 

3.  The  portal  circulation,  which  includes  the  portal  vein.  This  vein  is 
formed  by  the  union  of  the  radicles  of  the  gastric,  mesenteric,  and  splenic 
veins,  and  carries  the  blood  directly  into  the  liver,  where  the  vein  divides 
into  a  fine  capillary  plexus,  from  which  the  hepatic  veins  arise;  these 
empty  into  the  ascending  vena  cava. 

Orifices  and  Valves. — The  movement  of  the  blood  along  the  path  of 
the  circle  above  outlined  is  accomplished  by  the  alternate  contraction  and 
relaxation  of  the  muscle  walls  of  the  heart.  That  the  movement  may  be  a 
progressive  one,  that  there  shall  be  no  regurgitation  during  either  the 
contraction  or  the  relaxation,  it  is  essential  that  some  of  the  orifices  of 
the  heart  be  closed  during  each  of  these  periods.  This  is  accomplished 
by  the  heart  valves. 

The  valves  of  the  heart  are  formed  by  a  reduplication  of  the  endo- 
cardium strengthened  by  connective  tissue. 

The  right  auriculo-ventricular  opening  is  provided  with  a  valve  con- 
sisting of  three  portions  of  cusps  which  during  the  period  of  relaxation 
are  directed  into  the  ventricle;  during  the  contraction  they  are  raised  and 
placed  in  complete  apposition  when  they  act  as  a  valve  preventing  a 
backward  flow  into  the  auricle.  For  this  reason  it  is  known  as  the  tri- 
cuspid valve.  The  left  auriculo-ventricular  orifice  is  provided  with  a  valve 
consisting  of  but  two  cusps  and  is,  therefore,  termed  the  bicuspid  valve, 
or,  from  its  fancied  resemblance  to  a  bishop's  miter,  the  mitral  valve. 
The  mode  of  action  of  this  valve  is  similar  in  all  respects  to  the  tricuspid 
valve.     To  the  undersurface  and  to  the  edges  of  these  valves  the  tendinous 


CIRCULATION   OF   THE  BLOOD  I05 

cords  of  the  papillary  muscles  are  firmly  and  intricately  attached.  These 
cords  are  just  sufficiently  long  to  permit  closure  of  the  valves  and  to 
prevent  them  from  being  floated  into  the  auricle. 

The  orifice  of  the  pulmonic  artery  is  provided  with  three  semilunar  or 
pocket-shaped  membranes,  the  semilunar  valves.  The  orifice  of  the 
aorta  is  also  provided  with  three  similarly  arranged  semilunar  membranes, 
the  semilunar  valves. 

During  the  period  of  relaxation  of  the  heart  the  edges  of  the  semilunar 
valves  are  in  close  apposition  and  prevent  a  return  of  the  blood  into  the 
ventricles;  during  the  contraction  they  are  directed  into  the  pulmonic 
artery  and  aorta.  In  the  former  position  they  are  shut;  in  the  latter, 
they  are  open. 

The  Auriculo- ventricular  Bundle. — This  is  a  specialized  bundle  of 
muscle-fibers  discovered  in  part  by  His  and  in  part  by  Tawara  which 
unites  anatomically  and  physiologically  the  right  auricle  with  the  ven- 
tricles. The  reason  for  the  existence  of  this  bundle  lies  in  the  fact  that 
the  muscle  fibers  of  the  auricles  and  ventricles  are  completely  separated 
by  the  transverse  fibrous  septum  to  which  they  are  attached.  The  origin, 
course  and  distribution  of  this  bundle  is  as  follows: 

It  arises  near  the  opening  of  the  coronary  sinus  where  it  is  connected 
with  the  true  auricular  fibers.  From  their  origin  the  fibers  converge  to 
form  a  distinct  bundle  which  then  passes  forward  on  the  right  side  of  the 
auricular  septum  between  the  lower  edge  of  the  fossa  ovalis  and  the 
auriculo-ventricular  septum;  just  above  the  insertion  of  the  median  cusp 
of  the  tricuspid  valve  the  bundle  presents  a  very  complicated  network 
of  muscle-fibers  which  has  been  designated  as  a  knot  or  the  auriculo- 
ventricular  node  or  the  node  of  Tawara;  from  the  anterior  portion  of  the 
node  a  bundle  of  fibers  turns  downward  and  penetrates  the  auriculo- 
ventricular  septum,  beyond  which  it  passes  below  the  pars  membranacea 
septi  to  the  upper  limit  of  the  muscle  portion  of  the  ventricular  septum. 
It  then  divides  into  two  limbs  or  branches  which  descend  on  either  side 
of  the  septum  under  the  endocardium,  the  right  limb  lying  somewhat 
deeper  than  the  left.  Each  of  these  limbs  is  enclosed  by  a  layer  of  con- 
nective tissue  which  isolates  it  from  the  musculature  of  the  ventricular 
septum  as  far  as  the  lower  third  of  the  ventricular  cavities.  In  this 
region  they  divide  into  a  number  of  bundles,  some  of  which  enter  the 
papillary  muscles,  while  others,  forming  tendon-like  strands,  branch 
freely  beneath  the  endocardium  and  spread  in  all  directions  over  the 
entire  inner  surface  of  the  ventricle  and  enter  into  histologic  connection 
with  the  true  cardiac  muscle-fibers.  The  ultimate  terminations  of  this 
system   beneath   the  endocardium   constitutes   the   so-called   Purkinje- 


I06  HUMAN  PHYSIOLOGY 

fiber  layer.  From  its  function  this  bundle  has  been  termed  the  conduc- 
tion system  of  the  heart — the  conduction  of  an  excitation  process  from  the 
auricle  to  the  ventricles. 

The  Sino-auricular  Node  or  the  Keith-Flack  Node. — This  is  a  small  mass 
of  primitive  muscle  tissue  situated  in  the  sulcus  terminalis  at  the  junction 
of  the  superior  vena  cava  and  the  auricular  appendix.  It  is  supplied 
with  blood-vessels  and  nerves.  From  the  node  muscle-fibers  pass  along 
the  sulcus  for  a  distance  of  two  centimeters  and  finally  becomes  connected 
with  the  true  auricular  fibers.  Experimental  investigations  lead  to  the 
inference  that  it  is  directly  concerned  in  the  initiation  of  the  heart  beat. 
It  has  been  designated  the  "pace-maker"  of  the  heart. 

The  Mechanics  of  the  Heart.— With  each  beat,  the  heart  presents  two 
distinct  movements  which  alternate  with  each  other  in  quick  succession. 
One  is  the  movement  of  contraction,  or  the  systole,  by  which  the  blood 
contained  within  its  cavities  is  ejected  into  the  arteries — pulmonic  artery 
and  aorta;  the  other  is  the  movement  of  relaxation,  or  the  diastole,  fol- 
lowed by  a  pause  during  which  the  cavities  again  fill  up  with  blood  from 
the  venae  cavae  and  pulmonic  veins. 

The  contraction  of  any  part  of  the  heart  is  termed  the  systole;  the  re- 
laxation, the  diastole.  As  each  side  of  the  heart  has  two  cavities  the 
walls  of  which  contract  and  relax  in  succession,  it  is  customary  to  speak 
of  an  auricular  systole  and  diastole,  and  a  ventricular  systole  and  diastole. 
As  the  two  sides  of  the  heart  are  in  the  same  anatomic  relation  to  each 
other,  they  contract  and  relax  in  the  same  periods  of  time. 

The  immediate  cause  of  the  movement  of  the  blood  through  the  vessels 
is  the  contraction  and  relaxation  of  the  muscle-walls  of  the  heart,  and  more 
particularly  of  the  walls  of  the  ventricles,  each  of  which  plays  alternately 
the  part  of  a  force-pump,  and  possibly  to  a  slight  extent  of  a  suction-pump. 
-The  motive  power  is  furnished  by  the  heart  itself,  by  the  transformation 
of  potential  energy,  stored  up  during  the  period  of  rest,  into  kinetic  energy 
— i.e.,  heat  and  mechanic  motion. 

The  Cardiac  Impulse. — In  passing  from  the  diastolic  to  the  systolic 
condition  the  transverse  diameter  diminishes  while  the  antero-posterior 
diameter  increases,  and  the  whole  heart  becomes  somewhat  more  conic 
in  shape.  It  is  questionable  if  the  vertical  diameter  perceptibly  shortens. 
During  the  systole  the  heart  hardens,  increases  in  convexity,  and  is  more 
forcibly  pressed  against  the  chest  wall.  As  this  takes  place  suddenly, 
it  gives  rise  to  a  marked  vibration  of  the  chest  wall,  known  as  the  cardiac 
impulse. 

% 


CIRCULATION   Or   THE  BLOOD  I07 

This  impulse  is  principally  observed  in  the  space  between  the  fifth  and 
sixth  ribs  about  an  inch  internal  to  a  line  drawn  vertically  from  the 
middle  of  the  clavicle.  The  cardiac  impulse  is  synchronous  with  the 
cardiac  systole. 

The  Cardiac  Cycle. — The  term  cardiac  cycle  is  employed  to  express  the 
sequence  of  events  from  the  beginning  of  one  auricular  systole  and  the 
beginning  of  the  auricular  systole  which  immediately  follows  it.  An 
examination  of  the  heart  shows  that  each  pulsation  may  be  divided  into 
three  phases,  viz. : 

1.  The  auricular  systole. 

2.  The  ventricular  systole. 

3.  The  pause  or  period  of  repose  during  which  both  auricles  arid 
ventricles  are  at  rest. 

The  duration  of  a  cycle,  as  well  as  the  duration  of  its  three  stages,  varies 
in  different  animals  in  accordance  with  the  number  of  cycles  which  recur 
in  a  minute.  In  human  beings  in  adult  life  there  are  about  72  cycles  to 
the  minute;  the  average  duration  is,  therefore,  0.80  sec.  From  this  it 
follows  that  the  time  occupied  by  any  one  of  the  three  stages  must  be 
extremely  short  and  difficult  of  determination.  From  experiments  on 
animals  and  from  observations  made  on  human  beings,  the  following 
estimates  have  been  made  and  accepted  as  approximately  correct  for 
human  beings : 

1.  The  auricular  systole — 0.16  sec;  the  auricular  diastole,  0.64  sec. 

2.  The  ventricular  systole — 0.32  sec;  the  ventricular  diastole,  0.48  sec. 

3.  The  period  of  rest  for  both  auricles  and  ventricles — 0.32  sec. 

The  Movement  of  the  Blood  during  the  Cycle. — It  is  apparent  that 
with  the  relaxation  of  tlie  auricular  walls  blood  at  once  flows  from  the 
vanae  cavae  and  the  pulmonic  veins  into  the  auricular  cavities  and  continues 
so  to  do  throughout  the  entire  auricular  diastole.  With  the  relaxation  of 
the  ventricular  walls,  however,  the  blood  that  has  accumulated  in  the 
auricles  up  to  this  time,  or  its  equivalent  coming  from  the  venae  cavae 
and  pulmonic  veins,  now  flows  into  the  ventricles  until  they  are  nearly 
filled.  Before  they  are  filled,  however,  the  auricular  diastole  comes  to 
an  end,  the  auricular  walls  again  contract  and  force  some  of  their  con- 
tained blood  into  the  ventricles  and  thus  rapidly  complete  the  filling. 
The  ventricular  systole  immediately  follows,  during  which  the  blood  is 
driven  into  the  pulmonic  artery  and  aorta.  This  having  been  accom- 
plished, the  ventricles  relax,  and  the  blood  that  has  been  accumulating 
in  the  auricles  begins  to  flow  into  the  ventricles,  after  which  the  same 
series  of  events  follows  as  in  the  previous  cycle. 


I08  HUMAN  PHYSIOLOGY 

The  Action  of  the  Valves. — The  forward  movement  of  the  blood  is 
permitted  and  regurgitation  prevented  by  the  alternate  action  of  the 
auriculo- ventricular  and  semilunar  valves.  As  a  point  of  departure  for 
a  consideration  of  the  action  of  these  valves  and  their  relation  to  the 
systole  and  diastole  of  the  heart,  the  close  of  the  ventricular  systole 
may  be  selected.  At  this  moment,  the  semilunar  valves,  which  during 
the  systole,  are  directed  outward  by  the  blood  current  are  now  suddenly 
and  completely  closed  by  the  pressure  of  the  blood  in  the  aorta  and 
pulmonic  artery.     Regurgitation  into  the  ventricles  is  thus  prevented. 

During  the  ventricular  systole,  the  relaxed  auricles  are  filling  with 
blood.  With  the  ventricular  diastole  this  blood  or  its  equivalent  flows 
into  the  relaxed  and  easily  distensible  ventricles  until  both  auricles  and 
ventricles  are  nearly  filled.  The  tricuspid  and  mitral  valves  which  are 
hanging  down  into  the  ventricular  cavities,  are  now  floated  up  by  currents 
of  blood  welling  up  behind  them  until  they  are  nearly  closed.  The 
auricles  now  suddenly  contract,  forcing  their  contained  volumes  into  the 
ventricles  which  become  fully  distended. 

With  the  cessation  of  the  auricular  systole,  the  ventricular  systole 
begins.  If  the  blood  is  not  to  be  returned  to  the  auricles  the  tricuspid 
and  mitral  valves  must  be  instantly  and  completely  closed.  This  is 
accomplished  by  the  upward  pressure  of  the  blood  which  brings  their 
free  edges  in  close  apposition.  Reversal  in  the  position  of  these  valves 
is  prevented  by  the  contraction  of  the  papillary  muscles  which  exert  a 
traction  on  their  undersurfaces  and  edges  and  hold  them  steady. 

The  blood  now  confined  in  the  ventricles  between  the  closed  auriculo- 
ventricular  and  semilunar  valves  is  subjected  to  pressure  on  all  sides;  as 
the  pressure  rises  proportionately  to  the  vigor  of  the  contraction  there 
comes  a  moment  when  the  intra-ventricular  pressure  exceeds  that  in  the 
aorta  and  pulmonic  artery;  at  once  the  semilunar  valves  are  thrown  open 
and  the  blood  discharged.  Both  contraction  and  outflow  continue  until 
the  ventricles  are  practically  empty,  when  relaxation  sets  in  attended 
by  a  rapid  fall  of  pressure.  Under  the  influence  of  the  positive  pressure  of 
the  blood  in  the  aorta  and  pulmonic  artery,  the  semilunar  valves  are 
again  closed.  The  accumulation  of  blood  in  the  auricles,  attended  by  a 
rise  in  pressure,  again  forces  the  tricuspid  and  mitral  valves  open.  With 
these  events  the  cardiac  cycle  is  completed. 

Sounds  of  the  Heart. — If  the  ear  be  placed  over  the  cardiac  region, 
two  distinct  sounds  are  heard  during  each  revolution  of  the  heart,  closely 
following  each  other,  and  which  differ  in  character. 

The  sound  coinciding  with  the  systole  in  point  of  time — the  first  sound — 
is  prolonged  and  dull,  and  caused  by  the  closure  and  vibration  of  the 


CIRCULATION    OF   THE  BLOOD  lOQ 

auriculo-ventricular  valves,,  the  contraction  of  the  walls  of  the  ventricles, 
and  the  apex-beat;  the  second  sounds  occurring  at  the  beginning  of  the 
diastole,  with  the  second  phase  of  the  cardiac  cycle  is  short  and  sharp, 
and  caused  by  the  closure  and  vibration  of  the  semilunar  valves. 

The  Intra-ventricular  Pressures. — By  this  term  is  meant  the  pressure 
that  arises  in  the  ventricles  during  the  time  of  the  systole.  The  reason 
for  this  rise  of  pressure  arises  from  the  fact  that  the  semilunar  valves 
are  kept  tightly  shut  by  the  pressure  of  the  blood  in  the  aorta  and  pul- 
monic artery.  With  the  beginning  of  the  systole  the  auriculo-ventricular 
valves  are  suddenly  closed  and  now  the  blood  is  imprisoned.  If  the  semi- 
lunar valves  are  to  be  opened  and  the  blood  discharged  the  intra-ventricular 
pressure  must  exceed  the  pressure  in  the  aorta  and  pulmonic  artery. 
Moreover  as  the  aortic  and  pulmonic  pressures  increase  with  the  dis- 
charge of  blood  the  intra-ventricular  pressure  must  continue  to  rise  and 
exceed  the  increased  pressure  in  these  vessels.  This  the  heart  does  by 
calling  on  the  reserve  power  with  which  it  is  endowed.  Should  it  fail  to 
meet  some  sudden  rise  of  pressure  in  the  aorta  it  would  remain  in  a 
condition  of  permanent  diastole. 

The  Frequency  of  the  Heart-beat. — The  frequency  of  the  heart-beat 
varies  with  a  variety  of  conditions:  e.g.j  age,  sex,  posture,  exercise,  etc. 

Age. — The  most  important  normal  condition  which  modifies  the  activity 
of  the  heart  is  age.    Thus: 

Before  birth,  the  number  of  beats  a  minute  averages 140 

During  the  first  year  it  diminishes  to 128 

During  the  third  year  it  diminishes  to 95 

From  the  eighth  to  the  fourteenth  year  it  averages 84 

In  adult  males  it  averages    72 

Sex. — The  heart-beat  is  more  rapid  in  females  than  in  males.  Thus 
while  the  average  beat  in  males  is  72,  in  females  it  is  usually  8  or  lo  beats 
more. 

Posture. — Independent  of  muscle  efforts  the  rate  of  the  beat  is  influenced 
by  posture.  It  has  been  found  that  when  the  body  is  changed  from  the 
lying  to  the  sitting  and  to  the  standing  position,  the  beat  will  vary  as 
follows — from  66  to  71  to  81  on  the  average. 

Exercise  and  digestion  also  temporarily  increase  the  number  of  beats. 

A  rise  in  blood-pressure  from  any  cause  whatever  is  usually  attended  by 
a  decrease,  while  a  fall  in  blood-pressure  is  attended  by  an  increase  in  the 
rate. 

The  Blood  Supply  to  the  Heart. — The  nutrition  of  the  heart,  its  con- 
tractility, the  force  and  frequency  of  the  beat  are  dependent  on  and 


no  I  HUMAN   PHYSIOLOGY 

maintained  by  the  introduction  of  arterialized  blood  into  and  the  removal 
of  waste  products  from  its  tissue.  This  is  accomplished  by  the  coronary 
arteries  and  the  coronary  veins.  The  arteries  and  veins  on  the  surface 
of  the  heart  are  known  as  the  extra-mural  arteries  and  veins;  those  which 
are  found  in  the  substance  of  the  heart  are  known  as  intra-mural  arteries 
and  veins.  During  the  time  of  the  systole  the  intra-mural  branches  are 
.  compressed  and  the  blood  flow  into  the  heart  walls  interrupted,  though  at 
the  same  time  the  extra-mural  arteries  are  filled  with  blood  from  the  aorta; 
during  the  time  of  the  diastole,  the  recoil  of  these  latter  vessels  forces  the 
blood  into  the  intra-mural  arteries  and  capillaries,  thus  furnishing  to  the 
muscle  cells  an  additional  supply  of  nutritive  materials  and  receiving  prod- 
ucts of  waste;  at  the  succeeding  systole  the  venous  blood  is  driven  from 
the  intra-mural  into  the  extra-mural  veins  and  so  into  the  right  auricle. 

The  Causation  of  the  Heart-beat. — From  the  fact  that  the  heart  will 
continue  to  beat  for  a  variable  length  of  time  after  removal  from  the 
body  (the  time  varying  with  the  species  of  animal  from  which  it  has  been 
obtained)  it  is  evident  that  the  beat  is  independent  of  the  central  nerve 
system. 

The  fundamental  condition  for  the  continuance  of  the  beat  is  the  main- 
tenance of  the  irritability.  So  long  as  this  persists  the  heart  will  respond 
to  its  appropriate  stimulus.  The  irritability  of  the  heart  within  the  body 
is  dependent  on  the  supply  of  blood  coming  through  its  nutrient  vessels 
or  flowing  through  its  cavities.  Outside  the  body,  the  irritability  can 
be  maintained  for  some  hours  by  perfusing  the  coronary  system  of  vessels 
with  the  Ringer-Locke  solution. 

The  Nature  of  the  Stimulus. — The  presence  of  nerve-cells  in  the  walls  of 
the  heart,  their  relation  to  the  muscle  cells,  the  pronounced  activity  of  the 
sinus  of  the  frog  heart  where  they  are  very  abundant;  the  feeble  activity 
of  the  apex  where  they  are  absent  gave  rise  to  the  idea  that  the  stimulus  is 
a  nerve  impulse  rhythmically  and  automatically  discharged  by  these  nerve- 
cells.  This  view  is  no  longer  entertained.  It  has  been  demonstrated  that 
portions  of  the  heart  muscle,  that  do  not  contain  nerve-cells,  will  continue 
to  exhibit  rhythmic  contraction  for  some  hours  if  supplied  with  oxygenated 
and  defibrinated  blood;  that  the  embryonic  heart  contracts  rhythmically 
before  nerve-cells  have  migrated  to  its  walls. 

The  stimulus  therefore  evidently  arises  within  the  heart  muscle.  In 
other  words,  it  is  myogenic  and  not  neurogenic.  The  stimulus  is  now  be- 
lieved to  be  chemic  in  character  and  due  to  a  reaction  between  the 
inorganic  salts  in  the  muscle  cells  and  those  in  lymph  by  which  they  are 
surrounded. 


CIRCULATION   OF   THE  BLOOD  III 

The  Sequence  of  Sinus,  Auricular  and  Ventricular  Contraction. — An 

examination  of  the  heart  of  the  frog  shows  that  each  cycle  begins  with  a 
contraction  of  the  sinus  venosus  followed  by  a  contraction  of  the  auricle, 
then  by  a  contraction  of  the  ventricle.  Between  each  event  there  is  a 
definite  pause  the  result  of  an  interference  with  the  passage  of  the  excita- 
tion process  (this  being  the  cause  for  the  contraction)  across  the  junction 
of  these  cavities  formed  by  dense  connective  tissue.  The  difficulty  of 
passage  may  be  increased  by  ligation  of  the  sino-auricular  junction,  after 
which  the  auricles  or  ventricles  come  to  rest  though  the  sinus  beats  at 
the  usual  rate.  The  auriculo-ventricular  junction  may  also  be  ligated 
after  which  the  ventricle  comes  to  rest  though  the  sinus  and  auricles  beat 
as  usual.  The  physiologic  stimulus  apparently  arises  and  acts  in  the  walls 
of  the  sinus. 

In  the  mammal  the  visible  cycle  begins  by  a  contraction  of  the  auricle 
followed  by  that  of  the  ventricle.  Between  these  two  events  there  is  also 
a  definite  pause,  but  for  a  somewhat  different  reason.  In  the  mammal 
heart  the  cycle  begins  in  the  muscle  tissue  composing  the  node  of  Keith, 
the  so-called  pace-maker.  The  excitation  process  set  free  passes  to  the 
auricular  muscle  and  calls  forth  a  contraction.  Owing  to  the  fact  that 
there  is  no  continuity  of  muscle  fibers  across  the  junction,  there  has  been 
developed  the  auriculo-ventricular  bundle  previously  described,  which  has 
for  its  function  the  reception,  at  its  auricular  extremity,  of  the  excitation 
process  and  transmitting  it  to  the  ventricle  which  at  once  contracts  in 
response  to  its  stimulating  effect.  The  time  required  for  the  passage  of 
the  excitation  from  auricle  to  ventricle  is  about  0.2  sec.  and  is  known 
as  the  a-c  or  the  as-vs  interval. 

The  function  of  the  auriculo-ventricular  bundle  may  be  shown  by  its 
compression  by  a  suitable  clamp  or  transverse  section  in  the  heart  of  a 
mammal.  If  this  be  done  quickly  the  ventricle  at  once  ceases  to  beat, 
though  the  auricles  beat  at  their  accustomed  rate.  After  a  variable  period 
from  30  to  70  seconds,  the  ventricular  beat  returns  but  at  a  much  slower 
rate,  usually  about  one-third  the  auricular  rate.  The  cause  of  this  contrac- 
tion is  attributed  to  the  continuous  action  of  the  inorganic  salt,  in  the  blood. 
If  the  auriculo-ventricular  bundle  be  destroyed  by  a  pathologic  process 
in  that  portion  just  above  the  bifurcation  at  the  top  of  the  ventricular 
septum  (the  bundle  of  His)  the  ventricle  at  once  ceases  to  beat;  but  after 
the  usual  period  of  time  it  again  begins  to  beat,  but  at  a  slower  rate.  This 
is  the  underlying  cause  of  the  group  of  symptoms  known  as  the  Adams- 
Stokes  syndrome.  The  auricular  beats  as  indicated  by  the  pulsations  of 
the  jugular  veins  may  be  80  and  the  ventricular  beats  as  indicated  by  the 
pulse  from  22  to  30.  * 


112  HUMAN   PHYSIOLOGY 

The  Influence  of  the  Central  Nerve  System  on  the  Action  of  the 
Heart. — Though  the  heart-beat  is  independent  of  the  central  nerve  system, 
it  is  to  a  considerable  extent  modified  by  it  either  in  the  way  of  acceleration 
or  inhibition.  In  all  classes  of  animals  the  heart  not  only  contains  localized 
collections  of  nerve-cells,  but  it  is  also  connected  with  the  central  nerve 
systems  by  two  sets  of  nerve-fibers. 

In  the  frog  heart  a  group  of  nerve-cells  is  found  in  the  sinus  at  its  junc- 
tion with  the  auricle,  and  known  as  the  crescent  or  ganglion  of  Remak;  a 
second  group  is  found  at  the  base  of  the  ventricle  on  its  anterior  aspect  and 
known  as  the  ganglion  of  Bidder;  a  third  group  is  found  in  the  auricular 
septum,  known  as  the  septal  ganglion,  or  the  ganglion  of  Ludwig. 

In  the  dog  and  the  mammalian  heart  generally,  the  nerve-cells  though 
present  are  not  arranged  in  such  definite  groups,  but  are  distributed  in  the 
terminations  of  the  venae  cavae,  pulmonic  veins,  the  walls  of  the  auricles 
and  in  the  neighborhood  of  the  base  of  the  ventricles. 

These  cells  were  formerly  regarded  as  the  source  of  the  stimuli  for  the 
excitation  and  regulation  of  the  heart's  contraction.  This  view  is  no 
longer  entertained. 

The  extra  cardiac  nerves,  those  which  connect  the  heart  with  the 
central  nerve  system,  are  the  sympathetic  and  the  vagus.  Experiments 
have  demonstrated  that  the  sympathetic  is  the  motor  nerve  to  the  heart, 
the  nerve  that  accelerates  the  rate  and  augments  the  force  of  the  beat, 
while  the  vagus  is  the  inhibitor  nerve,  the  nerve  that  inhibits  or  controls 
the  rate  and  force  of  the  beat. 

Since  the  heart  muscle  belongs  to  the  autonomic  tissues,  it  follows  that 
the  accelerator  and  the  inhibitor  nerve  pathways  consist  of  two  consecu- 
tively arranged  neurones.  The  first  is  termed  preganglionic,  the  second 
postganglionic. 

The  Sympathetic. — The  preganglionic  fibers  have  their  origin  in  the 
medulla  oblongata  and  very  probably  from  nerve-cells  in  the  gray  matter 
beneath  the  floor  of  the  fourth  ventricle.  From  this  origin  they  descend 
the  spinal  cord  as  far  as  the  level  of  the  second,  third,  and  at  times  the 
fourth  thoracic  nerves.  At  this  level  they  emerge  from  the  cord  in  com- 
pany with  the  nerve-fibers  composing  the  anterior  roots  of  the  second, 
third,  and  fourth  thoracic  nerves.  After  a  short  course,  they  enter  the 
white  rami  communicantes,  then  the  sympathetic  chain  and  pass  upward 
to  the  ganglion  Stella tum  (the  first  thoracic),  and  to  the  inferior  cervical 
ganglion  as  well,  around  the  nerve-cells  of  both  of  which  their  terminal 
branches  arborize.  From  the  nerve-cells  of  both  the  stellate  and  inferior 
cervical  ganglia,  the  postganglionic  fibers  arise,  that  is,  the  sympathetic 
nerves  proper,  which  after  emerging  from  the  ganglia  pass  toward  the 


CIRCULATION   OF   THE  BLOOD  II3 

heart.  On  reaching  the  heart  they  terminate  directly  in  the  muscle- 
cell,  or  indirectly  through  the  intermediation  of  intra-cardiac  nerve-cells. 
The  former  mode  of  termination  is  the  more  probable. 

Stimulation  of  these  fibers  in  any  part  of  their  course,  more  readily  the 
sympathetic  fibers  after  their  emergence  from  the  ganglia,  is  followed  by 
an  increase  in  the  rate  and  sometimes  by  an  increase  in  the  force  of  the 
heart-beat.  For  this  reason  the  sympathetic  is  said  to  exert  an  accelerator 
and  an  augmentor  influence  on  the  heart-beat. 

The  percentage  increase  in  the  acceleration  varies  in  different  animals. 
In  some  instances  the  increase  varies  from  58  per  cent,  to  100  per  cent. 
If  the  heart  is  beating  slowly  before  stimulation,  the  acceleration  is  more 
marked  than  if  it  is  beating  rapidly. 

Division  of  the  sympathetic  nerves  is  at  once  followed  by  a  diminution 
in  the  rate,  the  degree  of  which  will  depend  to  some  extent  on  the  rate 
at  which  the  heart  was  beating  prior  to  the  division.  The  results,  there 
fore,  that  follow  stimulation  and  division  of  these  nerves  indicate  that 
they  are  transmitting  nerve  impulses  from  the  centers  from  which  they 
arise  to  the  heart,  upon  which  they  exert  a  stimulating  influence  on  the 
rate  and  force  of  the  beat. 

The  group  of  cells  from  which  the  accelerator  fibers  arise  is  known  as  the 
cardio-accelerator  center.  It  is  believed  to  be  in  a  state  of  continuous  or 
tonic  activity. 

The  Vagtis. — The  preganglionic  fibers  have  their  origin  in  a  group  of 
nerve-cells  situated  beneath  the  floor  of  the  fourth  ventricle.  From  this 
origin  they  pass  out  in  the  trunk-  of  the  vagus  proper.  In  the  neighbor- 
hood of  the  inferior  laryngeal  nerves,  branches  containing  efferent  fibers 
are  given  off  which  pass  to  the  heart.  Their  terminal  branches  arborize 
around  the  intra-cardiac  ganglia.  From  the  cells  of  the  ganglia  the  post- 
ganglionic fibers  arise  which  terminate  directly  in  some  of  the  heart  muscle 
fibers. 

Stimulation  of  the  vagus  fibers  in  any  part  of  their  course  with  induced 
electric  currents  will  cause  the  heart  to  come  to  a  standstill  almost  im- 
mediately in  the  condition  of  diastole,  and  may  be  so  kept  for  a  variable 
period,  from  fifteen  to  thirty  seconds  or  more,  during  which  its  walls  are 
relaxed  and  its  cavities  filled  with  blood.  On  cessation  of  the  stimulation 
the  contractions  return  and  in  a  very  short  time  the  former  rate  and  force 
of  the  beat  are  regained.  If  the  electric  currents  are  of  feeble  strength, 
the  heart  will  come  to  rest  gradually,  through  a  gradual  diminution  in  the 
rate  and  force  of  the  contraction.  During  the  period  of  inhibition  the 
walls  are  completely  relaxed  and  the  cavities  filled  with  blood. 

Division  of  one  vagus  is  followed  in  some  mammals,  e.g.,  dog  by  a 
8 


114  HUMAN  PHYSIOLOGY 

marked  increase  in  the  rate  of  beat  and  if  both  vagi  are  divided  the  increase 
may  amount  to  from  50  to  75  per  cent.  The  results  of  stimulation  and 
division  of  the  vagus  nerves  indicate  that  they  are  continuously  trans- 
mitting nerve  impulses  from  the  centers  from  which  they  arise,  to  the 
heart  muscle,  on  the  activity  of  which  they  exert  a  restraining  or  inhibitor 
influence. 

The  center  in  the  medulla  from  which  the  inhibitor  fibers  arise  is  known 
as  the  cardio-inhibitor  center.  This  center  is  also  believed  to  be  in  a 
state  of  continuous  activity  though  capable  of  being  increased  or  decreased 
in  activity  by  transmitted  nerve  impulses  from  various  regions  of  the 
body. 

Reflex  acceleration  or  inhibition  of  the  heart  is  caused  by  nerve  impulses 
transmitted  to  the  cardio-inhibitor  center  alone,  through  afferent  nerve- 
fibers,  some  of  which  are  inhibitor^  while  others  are  excitator.  In  the 
first  instance  the  center  is  inhibited  in  its  action  whereupon  the  cardio- 
accelerator  center  has  a  freer  action  and  the  heart  rate  is  accordingly 
accelerated;  in  the  second  instance  the  center  is  excited  to  increased 
activity  and  its  inhibitor  effect  increased.  The  effects  of  the  cardio- 
accelerator  center  is  thus  in  part  annulled  and  the  heart  rates  is  diminished. 

THE  VASCULAR  APPARATUS 

The  vascular  apparatus  in  its  entirely  consists  of  a  closed  system  of 
vessels  which  not  only  contain  the  blood,  but  under  the  driving  power  of 
the  heart,  transmit  it  to  and  from  all  regions  of  the  body.  It  is  usually 
divided  into  a  systemic  and  a  pulmonic  portion. 

The  Systemic  Vascular  Apparatus. — This  portion  of  the  general  vas- 
cular apparatus  includes  all  the  vessels  extending  f r(5m  the  left  ventricle  to 
the  right  auricle:  viz.,  the  arteries,  capillaries,  and  veins.  Though  serving 
as  a  whole  to  transmit  blood  from  the  one  side  of  the  heart  to  the  other, 
each  one  of  these  three  divisions  has  separate  but  related  functions,  which 
are  dependent  partly  on  differences  in  structure  and  physiologic  proper- 
ties, and  partly  on  their  relation  to  the  heart  and  its  physiologic  activities. 

The  Arteries. — The  arteries  serve  to  transmit  the  blood  ejected  from 
the  heart  to  the  capillaries;  that  this  may  be  accomplished  they  divide  and 
subdivide  and  ultimately  penetrate  each  and  every  area  of  the  body. 
Their  repeated  division  is  attended  by  a  diminution  in  size,  a  decrease  in 
the  thickness  and  a  change  in  the  structure  of  their  walls. 

A  typical  artery  consists  of  three  coats :  an  internal,  the  tunica  intima, 
a  middle,  the  tunica  media;  an  external,  the  tunica  adventitia. 

The  internal  coat  consists  of  a  structureless  elastic  basement  membrane, 
the  inner  surface  of  which  is  covered  by  a  layer  of  elongated  spindle-shaped 


CIRCULATION   OF   THE  BLOOD  II 5 

endothelial  cells.  The  middle  coat  consists  of  several  layers  of  circularly 
arranged,  non-striated  muscle-fibers,  between  which  are  networks  of 
elastic  fibers.  The  external  coat  consists  of  bundles  of  connective  tissue  of 
the  white  fibrous  and  yellow  elastic  varieties.  Between  the  external  and 
middle  coats  there  is  an  additional  elastic  membrane.  In  the  small 
arteries  there  is  but  a  single  layer  of  muscle-fibers.  In  the  large  arteries 
the  elastic  tissue  is  very  abundant,  exceeding  largely  in  amount  the 
muscle  tissue.  It  is  also  more  closely  and  compactly  arranged.  The 
external  coat  is  well  developed  in  the  large  arteries. 

The  presence  in  their  walls  of  both  elastic  and  contractile  elements, 
endows  the  arteries  with  the  two  properties  of  elasticity  and  contractility. 

Elasticity. — The  elasticity  is  best  developed  in  the  large  arteries,  though 
it  is  also  present  in  arteries  of  relatively  small  size.  By  virtue  of  the 
elasticity,  the  arteries  are  capable  of  being  distended  and  elongated  and 
when  the  distending  force  is  removed  of  recoiling  or  retracting  and  return- 
ing to  their  former  condition.  The  arteries  are  thus  enabled  to  adapt 
themselves  to  the  variations  in  the  volume  of  blood  discharged  from 
the  ventricle  at  a  single  beat  or  in  a  unit  of  time.  The  elasticity  also  con- 
verts the  intermittent  movement  of  the  blood  imparted  to  it  by  the  heart 
as  it  is  ejected  from  the  ventricle,  into  a  remittent  movement  in  the 
arteries  and  finally  into  the  continuous  and  equable  movement  observed 
in  the  capillaries. 

Contractility. — The  contractility,  especially  of  the  small  arteries, 
permits  of  a  variation  in  the  amount  of  blood  passing  into  a  given  capillary 
area  in  a  unit  of  time.  During  the  functional  activity  of  any  organ  or 
tissue  there  is  need  for  an  increase  in  the  amount  of  blood  beyond  that 
supplied  during  functional  inactivity  or  rest.  This  is  accomplished  by 
a  relaxation  of  the  muscle-fibers.  With  the  cessation  of  activity  the 
muscle-fibers  again  contract  and  reduce  the  amount  of  blood  to  that  re- 
quired for  nutritive  purposes  only.  The  tonic  contraction  of  the  arteriole 
muscle-fibers  increases  considerably  the  resistance  to  the  outflow  of  blood 
into  the  capillaries.  They  thus  assist  in  maintaining  the  blood-pressure 
in  the  arteries.  This  resistance  is  generally  termed  the  peripheral  resist- 
ance though  there  is  included  under  this  term  the  resistance  offered  by  the 
small  caliber  of  the  capillary  blood-vessel  as  well.  This  latter  factor  is 
constant,  the  former  variable. 

The  Capillaries. — The  capillaries  are  small  vessels  continuous  with  the 
arteries  on  the  one  hand  and  with  the  veins  on  the  other  hand.  Though 
different  in  structure  from  a  small  artery  or  vein,  there  is  no  sharp  boun  - 
dary  between  them,  as  their  structures  pass  imperceptibly  one  into  the 


Il6  HUMAN   PHYSIOLOGY 

other.  A  true  capillary,  however,  is  of  uniform  size  in  any  given  tissue 
and  does  not  undergo  any  noticeable  decrease  in  size  from  repeated 
branchings.  The  diameter  varies  in  different  tissues  from  0.0045  mm.  to 
0.0075  mm.,  just  sufficiently  large  to  perrait  the  easy  passage  of  a  single 
red  corpuscle.  The  length  varies  from  0.5  mm.  to  i  mm.  The  wall  of  the 
capillary  is  composed  of  a  single  layer  of  nucleated  endothelial  cells  with 
serrated  edges  united  by  a  cementing  material.  Though  extremely  short, 
the  capillaries  divide  and  subdivide  a  number  of  times,  forming  meshes  or 
networks,  the  closeness  and  general  arrangement  of  which  vary  in  different 
localities. 

As  the  endothelial  cells  are  living  structures  and  characterized  by  irrita- 
bility, contractility  and  tonicity,  it  may  be  assumed  that  the  capillary  wall 
as  a  whole  is  characterized  by  the  same  properties.  Upon  the  possession 
of  these  properties  the  functions  of  the  capillary  depend. 

The  function  of  the  capillary  vessel  is  to  permit  of  a  passage  of  the  nutri 
tive  materials  of  the  blood  into  the  surrounding  tissue  spaces  and  of  waste 
products  from  the  tissue  spaces  into  the  blood.  The  structure  of  the  capil- 
lary wall  is  well  adapted  for  this  purpose.  Composed  as  it  is  of  but  a  single 
layer  of  endothelial  cells,  the  thickness  of  which  defies  accurate  measure- 
ment, it  readily  permits,  under  certain  conditions,  of  the  necessary  ex- 
change of  materials  between  the  blood  and  the  tissues.  The  forces  which 
are  concerned  in  the  passage  of  materials  across  the  capillary  wall  are  em- 
braced under  the  terms  diffusion,  osmosis,  and  filtration.  As  a  result  of  the 
interchange  of  materials  the  tissues  are  provided  with  nutritive  materials 
and  relieved  of  the  presence  of  the  products  of  metabolism.  .As  the  blood 
loses  oxygen  and  gains  carbon  dioxid,  it  changes  in  color  from  a  scarlet  red 
to  a  bluish  red.  In  consequence  of  the  exchange  of  materials,  the  blood 
undergoes  a  change  in  composition,  the  extent  and  character  of  which 
varies  in  accordance  with  the  activities  and  character  of  the  organ  tra- 
versed by  it. 

In  order  that  the  nutritive  materials  may  pass  across  the  capillary 
wall  in  amounts  sufficient  to  maintain  the  necessary  supply  of  lymph  in  the 
lymph  or  tissue  spaces,  it  is  essential  that  the  blood  shall  flow  into  and  out 
of  the  capillary  vessels  constantly  and  equably,  in  volumes  varying  with 
the  activities  of  the  tissues,  under  a  given  pressure  and  with  a  definite 
velocity.  The  conditions  are  made  possible  by  the  cooperation  of  the 
physical  properties  and  actions  of  the  heart  and  vascular  apparatus. 

The  Veins. — The  veins  arise  from  the  distal  side  of  the  capillary  vessels. 
As  they  emerge  they  are  quite  small  and  designated  venules.  By  their 
convergence  and  union  the  veins  gradually  increase  in  size  in  passing  from 


CIRCULATION   OE   THE  BLOOD  II7 

the  periphery  toward  the  heart.  Their  walls  at  the  same  time  corre- 
spondingly increase  in  thickness.  The  veins  from  the  lower  extremities, 
the  trunk,  and  abdominal  organs  finally  terminate  in  the  inferior  vena 
cava.  The  veins  from  the  head  and  upper  extremities  terminate  in  the 
superior  vena  cava.     Both  venae  cavae  empty  into  the  right  auricle. 

The  veins  consist  of  three  coats,  an  internal,  a  middle  and  an  external 
similar  in  their  composition  to  most  of  the  arteries.  The  elastic  and 
muscular  tissues  are,  however,  not  so  abundant. 

Veins  are  distinguished  by  the  possession  of  valves  throughout  their 
course,  which  are  arranged  in  pairs,  and  formed  by  a  reflection  of  the  in- 
ternal coat,  strengthened  by  fibrous  tissues;  they  always  look  toward  the 
heart,  and  when  closed  prevent  a  reflux  of  blood  in  the  veins.  Valves 
are  most  numerous  in  the  veins  of  the  extremities,  but  are  entirely  absent  in 
many  others. 

The  Flow  of  the  Blood  through  the  Vessels. — During  the  flow  of  the 
blood  through  the  arteries,  capillaries  and  veins,  certain  phenomena  are 
presented  by  each  of  these  three  divisions  of  the  vascular  apparatus* 
These  are  mainly  velocity  and  pressure,  and  in  the  arteries  alone  an  alter- 
nate expansion  and  recoil  of  the  arterial  wall  with  each  heart-beat,  termed 
the  pulse. 

Blood -pressure. — Blood-pressure  may  be  defined  as  the  pressure  exerted 
radially  or  laterally  by  the  moving  blood  stream  against  the  sides  of  the 
vessels.  This  pressure  is  the  result  of  (i)  the  driving  power  of  the  heart, 
and  (2)  of  the  resistance  offered  to  the  forward  movement  of  the  blood — 
a  resistance  due  to  the  cohesion  and  friction  of  the  molecules  of  the  blood, 
of  the  blood  corpuscles,  and  the  adhesion  of  the  blood  to  the  sides  of  the 
blood-vessels.  That  there  is  such  a  pressure  within  the  arteries,  capil- 
laries, and  veins,  different  in  amount  in  each  of  these  three  divisions  of  the 
vascular  apparatus,  is  evident  from  the  results  which  follow  division  of  an 
artery  or  a  vein  of  corresponding  size.  When  an  artery  is  divided,  the 
blood  spurts  from  the  opening  for  a  considerable  distance  and  with  a  cer- 
tain velocity.  The  reason  for  this  lies  in  the  fact  that  the  vessel  has  been 
distended  by  the  pressure  from  within  and  its  walls  thrown  into  a  condition 
of  elastic  tension,  so  that  at  the  moment  there  is  an  outlet,  the  vessel 
suddenly  recoils  and  forces  the  blood  out  with  a  velocity  and  to  a  height 
proportional  to  the  distention.  When  a  vein  is  divided,  the  blood  as  a  rule 
merely  wells  out  of  the  opening  with  but  slight  momentum,  and  for  the 
reason  that  the  vessel  has  been  but  slightly,  if  at  all  distended  by  the  pres- 
sure. These  results  indicate  that  the  blood  in  the  arteries  stands  under 
a  pressure  considerably  higher  than  that  of  the  atmosphere,  while  that 


Il8  HUMAN  PHYSIOLOGY 

in  the  veins  stands  under  a  pressure  perhaps  but  slightly  above  that  of  the 
atmosphere.     Especially  true  is  this  of  the  larger  veins. 

Experimentally  it  has  been  determined  that  the  pressure  in  the  arteries 
at  the  end  of  the  cardiac  diastole  approximates  in  man  about  90  mm. 
Hg:  and  is  termed  the  diastolic  pressure.  During  the  systole  and  with 
the  discharge  of  blood  into  the  aorta  the  pressure  rises  from  30  to  40  mm. 
higher  which  is  then  termed  the  systolic  pressure.  The  pressure  in  the 
capillaries  approximates  20  to  40  mm.^and  in  the  veins  from  20  to  o  mm. 
or  even  less  at  the  terminations  of  the  venae  cavae. 

The  difference  in  the  height  of  the  pressure  in  the  venous  system  as 
contrasted  with  the  arterial  system  is  due  to  the  progressive  diminution  of 
the  resistance  from  the  beginning  of  the  aorta  to  the  ends  of  the  venae  cavae, 
together  with  the  small  diameter  of  the  capillaries,  increased  to  a  variable 
extent,  by  the  tonic  contraction  of  the  arteriole  muscle. 

The  Causes  of  the  Blood-pressure. — The  Heart.— The^  primary  factor 
in  the  production  of  the  pressure  is  the  pumping  action  of  the  heart. 
Should  there  be  any  cessation  in  its  activity,  the  elastic  walls  of  the  ar- 
teries would  recoil  and  force  the  blood  into  the  veins.  There  would  be 
coincidently  a  fall  of  the  pressure  to  that  of  the  atmosphere.  Even  under 
normal  circumstances  this  condition  is  approximated  during  the  diastole. 
The  recoil  of  the  arterial  wall  by  which  the  forward  movement  of  the 
blood  is  maintained  is  attended  by  a  fall  in  pressure.  But  before  this 
reaches  any  considerable  extent,  the  heart  again  contracts  and  forces  its 
contained  volume  of  blood  into  the  arteries. 

The  Resistance. — The  secondary  factor  is  the  resistance  to  the  flow 
of  blood  through  the  vessels,  the  nature  of  which  has  been  previously 
stated.  So  long  as  the  resistance,  and  especially  that  variable  element  of 
it  at  the  periphery  of  the  arterial  system,  viz.,  the  tonic  contraction  of  the 
arteriole  muscle  maintains  a  certain  average  value,  so  long  will  the  pres- 
sure in  each  division  of  the  vascular  apparatus  maintain  an  average  or  a 
physiologic  value.  Should  the  resistance  at  the  periphery  of  the  arterial 
system  vary  in  either  direction,  the  result  of  an  increase  or  a  decrease  in  the 
degree  of  the  contraction  of  the  arteriole  muscle,  there  will  arise  a  change 
in  the  relative  degree  of  pressure  in  each  of  the  three  divisions  of  the  vas- 
cular apparatus. 

The  Elasticity  of  the  Vessel  Walls, — A  tertiary  factor  is  the  elasticity  of 
the  arterial  wall.  While  it  can  hardly  be  said  that  the  elasticity  is  a  cause 
of  the  pressure,  there  can  be  attributed  to  it  the  capability  of  modifying 
and  assisting  in  the  maintenance  of  the  pressure  at  a  more  or  less  constant 
level;  for  were  it  not  for  this  property  of  the  vessel  wall  the  variations  in 
pressure  during  and  after  the  systole  would  be  far  more  extensive  than  they 


CIRCULATION   OF   THE  BLOOD  II 9 

are,  and  would  approximate  the  variations  observed  in  tubes  with  rigid 
walls.  The  elasticity,  moreover,  assists  in  the  equalization  of  the' blood 
stream,  converting  the  intermittent  and  remittent  flow  characteristic  of  the 
large  arteries  into  the  continuous  equable  stream  characteristic  of  the  cap- 
illaries. It  also  permits  of  wide  variations  in  the  amount  of  blood  the 
arteries  can  contain  between  their  minimum  and  maximum  distention. 

Variations  in  the  Arterial  Pressure. — From  the  preceding  statements 
it  is  apparent  that  the  existing  arterial  pressure  may  be  increased  by: 

1.  An  increase  in  the  rate  or  force  of  the  heart's  contraction. 

2.  An  increase  in  the  peripheral  resistance. 

3.  An  increase  in  both  the  force  of  the  heart  and  the  peripheral  resist- 
ance; and  it  is  further  apparent  that  if  the  pressure  is  higher  than  the  nor- 
mal it  may  be  lowered  to  the  normal  by  a  decrease  in  either  one  or  both  of 
these  factors. 

It  is  also  apparent  that  the  arterial  blood-pressure  as  a  whole  may  be 
decreased  below  the  normal  by: 

1.  A  decrease  in  the  rate  and  force  of  the  heart's  contraction. 

2.  A  decrease  in  the  peripheral  resistance. 

3.  A  decrease  in  both  the  force  of  the  heart  and  the  peripheral  resistance; 
and  it  is  again  further  apparent,  that  if  the  pressure  is  lower  than  the  nor 
mal  it  may  be  raised  to  the  normal  by  an  increase  in  either  one  or  both  of 
these  factors. 

The  Capillary  Pressure. — The  capillary  pressure,  though  possessing  an 
average  value,  may  be  increased  by  a  relaxation  of  the  arteriole  muscle  and 
decreased  by  their  contraction.  It  may  also  be  increased  by  any  inter- 
ference with  the  outflow  from  any  given  area,  or  decreased  by  factors  which 
favor  a  larger  outflow.  Independently  of. any  change  in  arteriole  resist- 
ance, a  rise  of  arterial  pressure  alone  will  increase  the  capillary  pressure. 

The  Venous  Pressure. — The  venous  pressure  as  a  whole  will  be  in- 
creased by  a 'fall  in  arterial  pressure  as  when  the  arterioles  relax  and  the 
heart  diminishes  in  force;  it  will  be  decreased  if  the  opposite  factors 
prevail. 

The  Pulse. — The  pulse  may  be  defined  as  a  periodic  expansion  and  recoil 
of  the  arterial  system.  The  expansion  is  caused  by  the  ejection  into  the 
arteries  of  a  volume  of  blood  during  the  systole;  the  recoil  is  due  to  the 
reaction  of  the  arterial  walls  on  the  blood  driving  it  forward  into  and 
through  the  capillaries,  during  the  diastole. 


120  HUMAN   PHYSIOLOGY 

At  the  close  of  the  cardiac  diastole  the  arteries  are  full  of  blood  and  con- 
siderably distended.  During  the  occurrence  of  the  succeeding  systole,  the 
incoming  volume  of  blood  is  accommodated  by  a  movement  forward  of  a 
portion  of  the  general  blood  volume  into  the  capillaries  and  a  further  dis- 
tention of  the  arteries.  The  distention  naturally  begins  at  the  beginning 
of  the  aorta.  As  the  blood  continues  to  be  discharged  from  the  heart,  ad- 
joining segments  of  the  aorta  expand  in  quick  succession  and  by  the  end  of 
the  systole  the  expansion  has  travelled  over  the  arterial  system  as  far  as  the 
capillaries.  This  expansion  movement  which  passes  over  the  arterial 
system  in  the  form  of  a  wave  is  known  as  the  pulse  wave,  or  the  pulse.  It 
is  this  alternate  expansion  and  recoil  which  is  perceived  by  the  finger  when 
placed  over  the  course  of  an  artery.  The  artery  best  adapted  for  this 
purpose  is  the  radial  as  it  passes  across  the  wrist-joint. 

The  Radial  Piilse. — If  the  ends  of  the  fingers  are  firmly  placed  over  the 
radial  artery,  not  only  the  increase  and  decrease  of  pressure,  but  also  many 
of  the  peculiarities  of  the  pulse-wave,  may  be  perceived.  Without  much 
difficulty  it  may  be  perceived  that  the  expansion  takes  place  quickly,  the 
recoil  relatively  slowly;  that  the  waves  succeed  one  another  with  a  certain 
frequency,  corresponding  to  the  heart-beat;  that  the  pulsations  are  rhyth- 
mic in  'character,  etc. 

Inasmuch  as  the  individuality  of  the  pulse-wave  varies  at  different 
periods  of  life  and  under  different  physiologic  and  pathologic  conditions, 
various  terms  more  or  less  expressive,  have  been  suggested  for  its  varying 
qualities.  Thus  the  pulse  is  said  to  be  frequent  or  infrequent  according  as 
it  exceeds  or  falls  short  of  a  certain  average  number — 72  per  minute; 
strong  or  weak  according  to  the  energy  with  which  the  vessel  expands; 
quick  or  slow,  according  co  the  suddenness  with  which  the  expansion  takes 
place  or  strikes  the  fingers;  hard  or  soft,  tense  or  easily  compressible ,  accord- 
ing to  the  resistance  which  the  vessel  offers  to  its  compression  by  the  fingers; 
large,  full  or  small,  according  to  the  volume  of  blood  ejected  into  the  aorta, 
or,  in  other  words,  the  degree  of  fullness  of  the  arterial  system. 

The  three  qualities  which  are  of  most  value  to  the  clinician  are  rate, 
strength  or  force,  and  volume. 

The  Velocity  of  the  Blood. — The  velocity  with  which  the  blood  flows  in 
the  arteries  diminishes  from  the  heart  to  the  capillaries,  owing  to  an  en- 
largement in  the  united  sectional  area  of  the  vessels;  the  velocity  increases 
from  the  capillaries  toward  the  heart  for  the  opposite  reason.  The  blood 
moves  most  rapidly  in  the  large  vessels,  and  especially  under  the  influence 
of  the  ventricular  systole.     From  experiments  on  animals,  it  has  been  es- 


CIRCULATION    OF   THE   BLOOD  121 

timated  to  move  in  the  aorta  of  man  at  the  rate  of  from  300  to  500  mm. 
asecond,andinthelarge  veins  at  the  rate  of  from  150  to  250  mm.  a  second, 
and  in  the  capillaries  from  0.5  to  i  mm.  per  second. 

The  Pulmonic  Vascular  Apparatus. — The  pulmonic  vascular  apparatus 
consists  of  a  closed  system  of  vessels  extending  from  the  right  ventricle 
to  the  left  auricle,  and  includes  the  pulmonic  artery,  capillaries,,  and  pul- 
monic veins.  In  its  anatomic  structure  and  physiologic  properties  it 
closely  resembles  the  systemic  apparatus. 

The  flow  of  the  blood  through  the  arteries,  capillaries  and  veins  is 
characterized  by  velocity  and  pressure  and  in  the  pulmonic  artery 
alone  by  the  presence  of  the  pulse.  The  causes  of  these  phenomena  in  the 
pulmonic  vascular  apparatus  are  the  same  as  in  the  systemic  apparatus. 
The  pressure  in  the  pulmonic  artery  of  the  dog  has  been  shown  by  Beutner 
to  be  about  one-third  that  in  the  aorta;  by  Bradford  and  Dean  to  be  one- 
fifth.  Wiggers  has  recently  shown  that  in  normally  breathing  dogs  with 
arterial  pressures  ranging  from  no  to  112  mm.  of  mercury,  the  maximal 
or  systolic  pressure  in  the  pulmonic  artery  averaged  36  mm.,  and  the 
minimal  or  diastolic  averaged  5  mm.  The  reason  for  the  low  pressure 
may  be  found  in  the  large  size  and  rich  development  of  the  pulmonic 
capillaries  and  the  imperfect  development  of  an  arteriole  muscle  at  the 
periphery  of  the  pulmonic  artery,  the  result  of  which  is  a  diminution 
in  the  friction.  Inasmuch  as  the  friction  is  relatively  low,  the  work  of 
the  right  heart  is  less  than  that  of  the  left  heart  and  hence  its  walls  are 
not  so  well  developed.  The  pulmonic  pressure  being  low  the  intra- 
ventricular pressure  of  the  right  heart  is  relatively  low  as  compared  with 
that  of  the  left  heart.  The  velocity  of  the  blood-stream  in  each  of  the 
three  divisions  of  the  system  cannot  well  be  determined.  The  time  oc- 
cupied by  a  particle  of  blood  in  passing  from  the  right  to  the  left  ventricle 
has  been  estimated  at  one-fourth  the  time  required  to  pass  from  the  left  to 
the  right  ventricle.  Assuming  the  latter  to  be  thirty  seconds,  the  former 
would  be  seven  and  one-half  seconds. 

The  capillary  vessels  are  spread  out  in  a  very  elaborate  manner  just 
beneath  the  inner  surface  of  the  pulmonic  air  cells,  and  form,  by  their 
close  relation  to  it,  a  mechanism  for  the  excretion  of  carbon  dioxid  and  the 
absorption  of  oxygen.  The  extent  of  the  capillary  surface  is  very  great. 
It  has  been  estimated  at  200  square  meters.  The  amount  of  blood  flowing 
through  this  system  hourly  and  exposed  to  the  respiratory  surface  is  about 
430  liters.  The  reason  for  the  existence  of  the  pulmonary  circulation  is  the 
renewal  of  the  oxygen  in  the  blood  and  the  elimination  of  the  carbon  dioxid; 
for  the  accomplishment  of  both  objects  ample  provision  is  here  made.     The 


122  HUMAN  PHYSIOLOGY 

flow  of  blood  through  the  cardio-pulmonic  vessels  is  subject  to  variation 
during  both  inspiration  and  expiration  in  consequence  of  their  relation  to 
the  respiratory  apparatus. 

Forces  Concerned  in  the  Circulation  of  the  Blood : 

1.  The  Contraction  of  the  Heart. — The  primary  forces  which  keep  the 
blood  flowing  from  the  beginning  of  the  aorta  to  the  right  side  of  the  heart 
and  from  the  beginning  of  the  pulmonary  artery  to  the  left  side  are  the 
contractions  of  the  left  and  right  ventricles  respectively.  Though  the 
heart's  energy  is  probably  sufficient  to  drive  the  blood  into  the  opposite 
side  of  the  heart,  it  is  supplemented  by  other  forces — e.g. : 

2.  Muscle  Contraction. 

3.  Thoracic  Aspiration. 

4.  The  Action  of  the  Valves  in  the  veins. 

The  Vaso-motor  Nerves. — These  are  nerves  that  impart  motor  activity 
to  the  muscle-fibers  of  the  arteriole  walls,  resulting  either  in  an  increase  or 
decrease  in  the  degree  of  their  contraction  and  thus  diminishing  or  increas- 
ing the  outflow  of  blood.  For  this  reason  they  are  termed  vaso-augmentor 
or  constrictor  nerves  and  vaso-inhibitor  or  dilatator  nerves. 

As  the  muscle-fibers  belong  to  the  autonomic  tissues,  the  nerve  supply  to 
them  consists  of  two  consecutively  arranged  neurons,  a  pre-ganglionic  and 
a  post-gangUonic. 

The  pre-ganglionic  vaso-constrictor  neurons  take  their  origin  from 
nerve-cells  located  in  the  anterior  horns  and  lateral  gray  matter  of  the 
spinal  cord.  They  emerge  from  the  cord  in  company  with  the  fibers  that 
compose  the  ventral  roots  of  the  spinal  nerves  from  the  second  thoracic  to 
the  second  or  third  lumbar  nerves  inclusive.  A  short  distance  from  the 
cord  they  leave  the  ventral  roots  as  the  white  rami  communicantes  and 
enter  for  the  most  part  the  vertebral  or  lateral  sympathetic  ganglia. 
From  the  results  of  many  observations  and  experiments  it  is  probable 
that  the  great  majority  of  the  vaso-constrictor  nerves  terminate  in  these 
ganglia;  that  is  to  say,  it  is  here  that  the  pre-ganglionic  fibers  arborize 
around  the  contained  nerve-cells.  From  the  nerve-cells  the  post-gangli- 
onic  fibers  arise,  which  pass  to  the  blood-vessels  of  (i)  the  body  walls; 
(2)  the  fore-limbs;  (3)  the  head,  neck  and  face;  (4)  the  hind  limbs;  and 
(5)  the  abdominal  viscera. 

The  fibers  for  the  blood-vessels  of  the  abdominal  viscera  and  which  are 
contained  in  the  trunk  of  the  splanchnic  nerve  pass  across  the  sympathetic 
chain  and  arborize  around  the  nerve-cells  in  the  semilunar  ganglion.  The 
post-ganglionic  arise  from  the  cells  of  this  ganglion  and  then  pass  to  the 
blood-vessels  of  the  stomach,  intestines,  liver,  etc. 


RESPIRATION  1^3 

The  Vaso-motor  Centers. — The  vaso -motor  centers  for  the  spinal  cord 
are  dominated  and  controlled  in  their  action  by  a  group  of  nerve-cells  in 
the  floor  of  the  fourth  ventricle  which  is  known  as  the  general  vaso-con- 
strictor  center.  This  center  is  supposed  to  consist  of  two  groups  of  cen- 
ters, viz.,  a  vaso-tonic  and  a  vaso-reflex  center;  the  former  maintains  the 
vascular  tonus  while  the  latter  permits  of  various  vaso-motor  reflexes. 

The  vaso-reflex  center  may  be  increased  or  decreased  in  its  activity  by 
nerve  impulses  transmitted  to  it  from  diflterent  regions  of  the  body,  in 
consequence  of  which  the  blood  distributed  to  larger  or  smaller  areas  of 
the  body  is  decreased  or  increased  in  accordance  with  their  physologic 
needs. 

Special  vaso-dilatator  centers  are  found  in  the  medulla,  for  the  blood- 
vessels of  the  glands  of  the  mouth,  nasal  chambers,  etc. ;  also  in  the  lower 
lumbar  region  of  the  cord  there  are  centers  for  the  blood-vessels  of  the 
sexual  organs.  Stimulation  of  these  centers,  either  reflexly,  or  directly 
from  the  cerebrum,  causes  dilatation  of  the  vessels  and  a  large  inflow  of 
blood. 

RESPIRATION 

Respiration  is  a  process  by  which  oxygen  is  introduced  into,  and  carbon 
dioxid  removed  from  the  body.  The  assimilation  of  the  former  and  the 
evolution  of  the  latter  take  place  in  the  tissues  as  a  part  of  the  general 
process  of  nutrition.  Without  a  constant  supply  of  oxygen  and  an  equally 
constant  removal  of  the  carbon  dioxid,  those  chemic  changes  which  under- 
lie and  condition  of  life  phenomena  could  not  be  maintained. 

The  general  process  of  respiration  may  be  considered  under  the  following 
headings,  viz.: 

1.  The  anatomy  and  general  arrangement  of  the  respiratory  apparatus. 

2.  The  mechanic  movements  of  the  thorax  by  which  an  interchange  of 
atmospheric  and  intra-pulmonary  air  is  accomplished. 

3.  The  chemistry  of  respiration;  the  changes  in  composition  undergone 
by  the  air,  blood,  and  tissues. 

4.  The  nerve  mechanism  by  which  the  respiratory  movements  are 
maintained  and  coordinated. 

The  Respiratory  Apparatus. — The  respiratory  apparatus  consists  essen- 
tially of : 

1.  The  lungs  and  the  air-passages  leading  into  them:  viz.,  the  nasal 
chambers,  mouth,  pharynx,  larynx,  and  trachea. 

2.  The  thorax  and  its  associated  structures. 


124  HUMAN   PHYSIOLOGY 

The  Larynx. — The  larynx  is  composed  of  firm  cartilages,  united  by  liga- 
ments and  muscles.  Running  anteroposteriorly  across  the  upper  opening 
are  four  ligamentous  bands — the  two  superior  oi  false  vocal  bands,  and  the 
two  inferior  or  true  vocal  bands — formed  by  folds  of  the  mucous  mem- 
brane. They  are  attached  anteriorly  to  the  thyroid  cartilages  and  pos- 
teriorly to  the  arytenoid  cartilages,  and  are  capable  of  being  separated  by 
the  contraction  of  the  posterior  crico-arytenoid  muscles,  so  as  to  admit 
the  passage  of  air  into  and  from  the  lungs. 

The  Trachea. — The  trachea  is  a  tube  from  lo  to  12  centimeters  in 
length,  2  centimeters  in  diameter,  extending  from  the  cricoid  cartilage  of 
the  larynx  to  the  fifth  thoracic  vertebra,  where  it  divides  into  the  right 
and  left  bronchi.  It  is  composed  of  a  series  of  cartilaginous  rings,  which 
extend  about  two-thirds  around  its  circumference,  the  posterior  third 
being  occupied  by  transversely  arranged  non-striated  muscle-fibers  known 
as  the  tracheal  muscle.  Being  attached  to  the  ends  of  the  cartilages  it  is 
capable,  by  alternately  contracting  and  relaxing,  of  diminishing  or  increas- 
ing the  lumen  of  the  trachea.  Opposite  the  fifth  thoracic  vertebra  the 
trachea  divides  into  a  right  and  left  bronchus.  Each  bronchus  then 
subdivides  into  two  other  branches  which  penetrate  the  corresponding  lung 
about  the  middle  of  the  inner  surface. 

The  Lungs. — The  lungs,  in  the  physiologic  condition,  occupy  the 
greater  part  of  the  cavity  of  the  thorax.  They  are  separated  from  each 
other  by  the  contents  of  the  mediastinal  space:  viz.,  the  heart,  the  large 
blood  vessels,  the  esophagus,  etc. 

A  histologic  analysis  of  the  lung  shows  it  to  consist  of  the  branches  of  the 
bronchi,  their  subdivisions  and  ultimate  terminations,  blood-vessels, 
lymphatics  and  nerves,  imbedded  in  a  stroma  of  fibrous  and  elastic  tissue. 
The  anatomic  relations  which  these  structures  bear  one  to  another  is  as 
follows : 

Within  the  substance  of  the  lung  the  bronchi  divide  and  subdivide, 
giving  origin  to  a  large  number  of  smaller  branches,  the  bronchial  tubes, 
which  penetrate  the  lung  in  all  directions  (Fig.  12).  With  this  repeated 
subdivision  the  tubes  become  narrower,  their  walls  thinner,  their  structure 
simpler.  In  passing  from  the  larger  to  the  smaller  tubes  the  cartilaginous 
arches  become  shorter  and  thinner,  and  finally  are  represented  by  small 
angular  and  irregularly  disposed  plates.  In  the  smallest  tubes  the 
cartilage  entirely  disappears.  With  the  diminution  of  the  caliber  of  the 
tube  and  a  decrease  in  the  thickness  of  its  walls,  there  appears  a  layer  of 
non-striated  muscle-fibers,  the  so-called  bronchial  muscle,  between  the 
mucous  and  submucous  tissues,  which  completely  surrounds  the  tube  and 


RESPIRATION 


125 


becomes  especially  well  developed  in  those  tubes  devoid  of  cartilage. 
The  fibrous  and  mucous  coats  at  the  same  time  diminish  in  thickness. 
When  the  bronchial  tube  has  been  reduced  to  the  diameter  of  about  one 
millimeter,  it  is  known  as  a  bronchiole  or  a  terminal  bronchus.  From  the 
sides  of  the  terminal  bronchus  and  from  its  final  termination  there  is  given 
off  a  series  of  short  branches  which  sooxpan  end  to  form  lobules  or  aleveoli. 
The  cavity  of  the  alveolus  is  termed  the  infundibulum.  From  the  inner 
surface  of  the  alveolus  and  of  the  passageway  leading  into  it,  there  project 

thin  partitions  which  subdivide  the 
outer  portion  of  the  general  cavity  or 
infundibulum  into  small  spaces,  the 
so-called  air-sacs  or  air-cells.  The  wall 
of  the  alveolus  is  extremely  thin  and 
consists  of  fibro-elastic  tissue,  support- 
ing a  very  elaborate  capillary  net- 
work of  blood-vessels.  The  bronchial 
system  as  far  as  the  alveolar  passages  is 
lined  by  ciliated  epithelium.  The  air- 
sacs  are  lined  by  flat  epithelial  plates 
of  irregular  shape,  termed  the  respira- 
tory epithelium.  The  alveoli  are  united 
one  to  another  by  fibro-elastic  tissue. 

Bronchial  Innervation. — The  bron- 
chial muscles  are  presumably  in  a  state 
of  tonic  contraction  and  impart  to  the 
bronchial  tubes  a  certain  average  caliber 
best  adapted  for  respiratory  purposes. 
Experimental  investigations  indicate 
that  they  are  innervated  by  efferent 
fibers  of  the  vagus  nerve  (broncho- 
constrictors  and  possibly  broncho-dila- 
tators) inasmuch  as  stimulation  of  this  nerve  is  usually  followed  by  a  con- 
traction of  the  muscles  and  a  narrowing  of  the  lumen  of  the  bronchial 
system.  These  muscles  may  also  be  thrown  into  increased  acitivity  by 
the  inhalation  of  irritating  gases  and  into  a  tetanus  by  pathologic  causes 
as  seen  in  the  various  forms  of  asthma. 

The  Pulmonic  Blood-vessels. — The  two  main  divisions  of  the  pulmonic 
artery  distribute  the  venous  blood  to  the  pulmonic  lobules.  As  the  lobules 
are  approached  a  small  arterial  branch  plunges  into  the  wall  of  the  lobule, 
in  which  its  branches  form  a  rich  capillary  network  in  which  surrounds  and 


Fig.  12. — Diagram  of  the  Re- 
spiratory Organs. 
The  windpipe,  leading  down 
from  the  larynx,  is  seen  to  branch 
into  two  large  bronchi,  which 
subdivide  after  they  enter  their 
respective  lungs. 


126  HUMAN  PHYSIOLOGY 

embraces  the  air  sacs  on  all  sides.  The  blood  emerging  from  the  capil- 
laries is  conducted  by  the  converging  system  of  veins — the  pulmonic 
veins — into  the  left  auricle  of  the  heart.  The  main  function  of  the  pul- 
monic apparatus  and  the  pulmonic  division  of  the  circulatory  apparatus 
is  to  afford  a  ready  means  for  the  exhalation  of  the  carbon  dioxid  and  the 
absorption  of  oxygen.  In  consequence  of  this  exchange  of  gases  the 
blood  changes  in  color  from  dark  bluish-red  to  scarlet  red. 

The  Thorax. — The  thorax  in  which  the  respiratory  organs  are  lodged,  is 
of  a  conic  shape,  having  its  apex  directed  upward,  its  base  downward.  Its 
framework  is  formed  posteriorly  by  the  spinal  column,  anteriorly  by  the 
sternum,  and  laterally  by  the  ribs  and  costal  cartilages.  Between  and 
over  the  ribs  lie  muscles,  fascia,  and  skin;  above,  the  thorax  is  completely 
closed  by  the  structures  passing  into  it  and  by  the  cervical  fascia  and  skin ; 
below,  it  is  closed  by  the  diaphragm.     It  is,  therefore,  an  air-tight  cavity. 

The  Pleura. — Each  lung  is  surrounded  by  a  closed  serous  membrane 
(the  pleura),  one  layer  of  which  (the  visceral)  is  reflected  over  the  lung; 
the  other  (the  parietal),  reflected  over  the  wall  of  the  thorax;  between  the 
two  layers  is  a  small  amount  of  fluid,  which  prevents  friction  during  the 
play  of  the  lungs  in  respiration. 

The  Relation  of  the  Respiratory  Organs. — Intra-pulmonic  pressure. — 
When  the  thorax  is  in  a  condition  of  rest,  as  at  the  end  of  an  expiration  the 
lungs  are  full  of  air  and  by  reason  of  their  distensibility  completely  fill  all 
portions  of  the  thorax  not  occupied  by  the  heart,  great  vessels,  and  esopha- 
gus. This  condition  is  maintained  by  the  pressure  of  the  air  in  the  lungs, 
the  intra-pulmonic  pressure,  which  is  that  of  the  atmoshpere  760  mm.  Hg. 
This  relation  persists  so  long  as  the  thorax  remains  air  tight.  If,  however, 
an  opening  be  made  in  the  thoracic  wall,  the  lung  immediately  collapses 
and  a  pleural  cavity  is  established.  The  pressure  of  air  within  and  without 
the  lung  counterbalancing,  at  the  moment  the  air  is  admitted,  the  elastic 
tissue  at  once  recoils  and  forces  a  large  part  of  the  air  out  of  the  lung. 
This  is  a  proof  that  in  the  normal  condition,  the  lungs,  distended  by  atmos- 
pheric pressure  from  within,  are  in  a  state  of  elastic  tension  and  ever  en- 
deavoring to  pull  the  pulmonic  layer  of  the  pleura  away  from  the  parietal 
layer.  That  they  do  not  succeed  in  doing- so  is  due  to  the  fact  that  the 
atmospheric  pressure  from  without  is  prevented  from  acting  on  the  lung 
by  the  firm  unyielding  walls  of  the  thorax. 

Intra-thoracic  Pressure. — As  a  result  of  the  elastic  tension  of  the  lungs  a 
fractional  part  of  the  intra-pulmonic  pressure,  760  mm.  Hg.,  is  counter- 
balanced or  opposed,  so  that  the  heart  and  great  vessels  and  other  intra- 
thoracic viscera  are  subjected  to  a  pressure  somewhat  less  than  that  of  the 


RESPIRATION  1 27 

atmosphere;  the  amount  of  this  pressure  will  be  that  of  the  atmosphere 
less  that  exerted  by  the  elastic  tissue  of  the  lung  in  the  opposite  direction, 
expressed  in  terms  of  millimeters  of  mercury.  In  the  thorax,  but  outside 
the  lungs,  there  then  prevails  a  pressure,  negative  to  the  pressure  inside  the 
lungs  and  which  is  known  as  the  intra-thoracic  pressure. 

The  elastic  tension  of  the  lung  has  been  determined  for  the  human  lung 
and  amounts  to  about  6  mm.  Hg.  The  intra-thoracic  pressure  is  nega- 
tive to  the  intra-pulmonic  pressure  by  6  mm.  Hg. 

The  Respiratory  Movements. — As  the  blood  flows  through  the  pul- 
monic capillaries  it  yields  carbon  dioxid  to,  and  receives  oxygen  from,  the 
air  in  the  pulmonic  alveoli.  As  a  result,  the  intra-pulmonic  air  changes  in 
composition,  which  interferes  to  a  greater  or  less  extent  with  the  further  ex- 
change of  gases.  That  this  exchange  may  continue,  it  is  of  primary  impor- 
tance that  the  air  within  the  alveoli  be  renewed  as  rapidly  as  it  is  vitiated. 
This  is  accomplished  by  an  alternate  increase  and  decrease  in  the  capacity 
of  the  thorax,  accompanied  by  corresponding  changes  in  the  capacity  of  the 
lungs.  During  the  former  there  is  an  inflow  of  atmospheric  air  (inspira- 
tion), during  the  latter  an  outflow  of  intra-pulmonic  air  (expiration). 
The  continuous  recurrence  of  these  two  movements  brings  about  that  de- 
gree of  pulmonic  ventilation  necessary  to  the  normal  exchange  of  gases 
between  the  blood  and  the  air.  The  two  movements  together  constitute 
a  respiratory  act  or  cycle. 

1.  Inspiration  is  an  active  process,  the  result  of  the  expansion  of  the 
thorax,  whereby  the  atmospheric  air  is  introduced  into  the  lungs. 

2.  Expiration  is  a  partially  passive  process,  the  result  of  the  recoil  of  the 
elastic  walls  of  the  thorax,  and  the  recoil  of  the  elastic  tissue  of  the  lungs 
whereby  the  intrapulmonary  air  is  expelled. 

In  inspiration  the  chest  is  enlarged  by  an  increase  in  all  its  diameters — 
viz.: 

1.  The  vertical  is  increased  by  the  contraction  and  descent  of  the 
diaphragm. 

2.  The  anteroposterior  and  transverse  diameters  are  increased  by  the  ele- 
vation and  rotation  of  the  ribs  upon  their  axes. 

In  ordinary  tranquil  inspiration  the  muscles  which  elevate  the  ribs  and 
thrust  the  sternum  forward,  and  so  increase  the  diameters  of  the  chest,  are 
the  external  intercostals,  running  from  above  downward  and  forward;  the 
sternal  portion  of  the  internal  intercostals,  and  the  levatores  costarum. 

In  the  extraordinary  efforts  of  inspiration  certain  auxiliary  muscles  are 
brought  into  play — viz.,  the  sternomastoid,  pectorales,  serratus  magnus — 
which  increase  the  capacity  of  the  thorax  to  its  upmost  limit. 


128  HUMAN  PHYSIOLOGY 

In  expiration  the  diameters  of  the  chest  are  all  diminished — viz.: 

1.  The  vertical,  by  the  ascent  of  the  diaphragm. 

2.  The  anteroposterior,  by  a  depression  of  the  ribs  and  sternum. 

In  ordinary  tranquil  expiration  the  diameters  of  the  thorax  are  diminished 
by  the  recoil  of  the  elastic  tissue  of  the  lungs  and  the  ribs;  but  in  forcible 
expiration  the  muscles  which  depress  the  ribs  and  sternum,  and  thus  further 
diminish  the  diameter  of  the  chest,  are  the  internal  intercostals,  the  infra- 
costals,  and  the  triangularis  sterni. 

In  the  extraordinary  efforts  of  expiration  certain  auxiliary  muscles  are 
brought  into  play — viz.,  the  abdominal  and  sacrolumhalis  muscles — which 
diminish  the  capacity  of  the  thorax  to  its  utmost  limit. 

The  Movements  of  the  Lungs. — By  reason  of  the  distensibility  and  the 
elastic  recoil  of  the  lungs,  they  follow  all  variations  in  the  size  of  the  thorax 
enlarging  during  inspiration  to  accommodate  the  incoming  volume  of  air 
and  diminishing  during  expiration  to  assist  in  the  removal  of  a  correspond- 
ing amount  of  air. 

During  the  enlargement  of  the  thorax,  the  intra-pulmonic  air  expands 
and  its  pressure  falls  in  consequence  of  which  the  atmospheric  air  rushes  in 
to  restore  atmospheric  pressure.  Coincidently  the  lungs  are  expanded  and 
kept  in  close  contact  with  the  thoracic  walls  and  the  diaphragm.  During 
the  diminution  in  the  size  of  the  thorax,  the  intra-pulmonic  air  is  com- 
pressed and  its  pressure  rises,  in  consequence  of  which  the  intra-pulmonic 
air  rushes  out  through  the  air  passages  until  the  atmospheric  pressure  is 
reached.  Coincidently  the  elastic  recoil  of  the  lungs  restores  them  to  their 
former  size  and  volume. 

The  intra-thoracic  pressure  falls  during  inspiration  and  rises  during  ex- 
piration. The  expansion  of  the  lungs  is  attended  by  an  increase  in  the 
elastic  recoil  and  hence  a  neutralization  of  a  larger  percentage  of  the  intra- 
pulmonic  pressure.  The  recoil  of  the  lungs  during  expiration  has  the  oppo- 
site result. 

The  fall  of  intra-thoracic  pressure  has  a  favorable  influence  on  the  flow  of 
blood  from  the  extra-thoracic  veins  into  the  intra-thoracic  veins,  the  right 
side  of  the  heart  and  the  cardio-pulmonic  vessels.  The  flow  of  lymph  from 
the  lower  portion  of  the  thoracic  duct  into  the  upper  portion  is  also  in- 
creased. During  expiration  the  reverse  movement  is  prevented  by  the 
action  of  the  valves. 

Types  of  Respiration. — Observations  of  the  respiratory  movements  in 
the  two  sexes  shows  that  while  the  enlargement  of  the  thoracic  cavity  is 
accomplished  both  by  the  descent  of  the  diaphragm  (as  shown  by  the  pro- 
trusion of  the  abdomen)  and  the  elevation  of  the  thoracic  walls,  the  former 


RESPIRATION  1 29 

movement  preponderates  in  the  male,  the  latter  in  the  female,  giving  rise 
to  what  has  been  termed  in  the  one  case  the  diaphragmatic  or  abdominal 
type  and  in  the  other  the  thoracic  or  costal  type  of  respiration.  Modern 
methods  of  investigations  have  established  the  view  that  the  preponder- 
ance of  thoracic  movement  is  due  to  the  influences  of  dress  restrictions, 
for  with  their  removal  the  so-called  costal  type  of  breathing  entirely 
disappears.  While  gestation  may  lead  to  a  greater  activity  of  the  thorax, 
this  is  but  temporary,  for  with  its  termination  there  is  a  return  to  the  dia- 
phragmatic type  of  breathing. 

Number  of  Respirations  per  Minute. — The  number  of  respirations  which 
occur  in  a  unit  of  time  varies  with  a  variety  of  conditions,  the  most  impor- 
tant of  which  is  age.  The  results  of  the  observations  of  Quetelet  on  this 
point,  which  are  generally  accepted,  are  as  follows : 

Respirations  Respirations 

Age  per  minute  Age  per  minute 

0-  I  year 44  20-25  years 18.7 

5  years 26  25-30  years iS  •  O 

15-20  years 20  30-50  years 170 

From  these  observations  it  may  be  assumed  that  the  average  number  of 
respirations  in  the  adult  is  eighteen  per  minute,  though  varying  from 
moment  to  moment  from  sixteen  to  twenty.  During  sleep,  however,  the 
respiratory  movements  often  diminish  in  number  as  much  as  30  per  cent., 
at  the  same  time  diminishing  in  depth. 

Volumes  of  Air  Breathed. — The  volumes  of  air  which  enter  and  leave 
the  lungs  with  each  inspiration  and  expiration  naturally  vary  with  extent 
of  the  movement,  though  four  volumes  at  least,  may  be  determined* 
(i)  that  of  an  ordinary  inspiration;  (2)  that  of  an  ordinary  expiration; 
(3)  that  of  a  forced  inspiration;  (4)  that  of  a  forced  expiration. 

By  means  of  the  spirometer  the  amount  of  the  foregoing  four  volumes 
have  been  determined  and  named  as  follows: 

1 .  The  tidal  volume,  that  which  flows  into  and  out  of  the  lungs  with  each 
inspiration  and  expiration,  which  varies  from  20  to  30  cubic  inches  (330  to 
500  c.c). 

2.  The  complemental  volume,  that  which  flows  into  the  lungs,  in  addition 
to  the  tidal  volume,  as  a  result  of  a,  forcible  inspiration,  and  which  amounts 
to  about  no  cubic  inches  (1,800  c.c). 

3.  The  reserve  volume,  that  which  flows  out  of  the  lungs,  in  addition  to 
the  tidal  volume,  as  a  result  of  a  forcible  expiration,  and  which  amounts  to 
about  100  cubic  inches  (1,650  c.c). 

After  the  expulsion  of  the  reserve  volume  there  yet  remains  in  the  lungs 


130  HUMAN   PHYSIOLOGY 

an  unknown  volume  of  air  which  serves  the  mechanic  function  of  distend- 
ing the  air-cells  and  alveolar  passages,  thus  maintaining  the  conditions 
essential  to  the  free  movement  of  blood  through  the  capillaries  and  to  the 
exchanges  of  gases  between  the  blood  and  alveolar  air.  As  this  volume  of 
air  cannot  be  displaced  by  volitional  effort,  but  resides  permanently  in  the 
alveoli  and  bronchial  tubes  though  constantly  undergoing  renewal,  it  was 
termed — 

4.  The  residual  volume,  the  amount  of  which  is  difficult  of  determina- 
tion, but  has  been  estimated  by  different  observers  at  914  c.c.  1,562  c.c, 
1,980  c.c. 

The  Vital  Capacity  of  the  Lungs. — The  total  volume  of  the  air  in  the 
lungs  at  the  time  of  their  maximum  distention  represents  the  vital  capacity 
in  the  physiological  condition  and  includes  the  tidal,  the  complemental, 
the  reserve  and  the  residual  air.  The  vital  capacity,  however,  has  been 
defined  as  the  amount  of  air  which  can  be  expelled  by  the  most  forcible 
expiration  after  the  most  forcible  inspiration,  this  therefore  excludes  the 
residual  volume.  The  vital  capacity  was  supposed  to  be  an  indication  of 
an  individual's  respiratory  power,  not  only  in  physiologic  but  also  in  patho- 
logic conditions.  Though  averaging  about  230  cubic  inches  (3,770  c.c.; 
for  an  individual  5  feet  7  inches  in  height,  the  vital  capacity  varies  with  a 
number  of  conditions,  the  most  important  of  which  is  stature.  It  is 
found  that  between  5  and  6  feet  the  capacity  increases  8  inches  (130  c.c.) 
for  each  inch  increase  in  height. 

The  total  volume  of  air  breathed  daily  can  be  approximately  determined 
by  multiplying  the  average  volume  of  air  taken  in  at  one  inspiration  and 
multiplying  by  the  number  of  respirations  per  minute.  Assuming  that 
an  individual  takes  into  the  lungs  at  each  inspiration  330  to  500  c.c.  (20 
to  30  cubic  inches)  and  at  the  same  time  breathes  18  times  per  minute 
there  would  pass  into  the  lungs  during  the  twenty-fours,  8,500  to  12,752 
liters. 

The  Chemistry  of  Respiration.  Changes  in  the  Composition  of  the 
Air  Breathed. — Experience  teaches  that  the  air  during  its  sojourn  in  the 
lungs  undergoes  such  a  change  in  composition  that  it  is  rendered  unfit  for 
further  breathing.  Chemic  analysis  has  shown  that  this  change  involves 
a  loss  of  oxygen,  a  gain  in  carbon  dioxid,  watery  vapor,  and  organic  matter. 
For  the  correct  understanding  of  the  phenomena  of  respiration  it  is  essen- 
tial that  not  only  the  character  but  the  extent  of  these  changes  be  known. 
This  necessitates  an  analysis  of  both  the  inspired  and  expired  airs,  from  a 
comparison  of  which  certain  deductions  can  be  made. 


RESPIRATION  I3I 

The  results  which  have  been  obtained  are  represented  in  the  following 
table : 


Inspired  air 

f  Oxygen 20.80.         ^^^ 

100  J  Carbon  dioxid traces.  . 

,     <  ^,.  vols, 

vols.     Nitrogen 79  •  20. 

[  Watery  vapor    variable. 


Expired  air 

Oxygen 16.02. 

Carbon  dioxid 4-38. 

Nitrogen 79  •  60. 

Watery  vapor saturated. 

Organic  matter a  trace. 


These  analyses  indicate  that  under  ordinary  conditions  the  air  loses 
oxygen  to  the  extent  of  4.78  per  cent,  and  gains  carbon  dioxid  to  the  extent 
of  4.38  per  cent.;  that  it  gains  in  nitrogen  to  the  extent  of  0.4  per  cent,  and 
in  watery  vapor  from  its  initial  amount  to  the  point  of  saturation,  as  well 
as  in  organic  matter.  It  is  to  these  changes  in  their  totality  that  those 
disturbances  of  physiologic  activity  are  to  be  attributed  which  arise  when 
expired  air  is  re-breathed  for  any  length  of  time  without  having  undergone 
renovation.  From  the  percentage  loss  of  oxygen  and  gain  in  carbon  dioxid 
the  total  oxygen  absorbed  and  carbon  dioxid  exhaled  may  be  approximately 
calculated.  Thus,  if  the  volume  of  air  breathed  daily  be  accepted  at  either 
8,500  or  12,752  liters,  and  the  percentage  loss  of  oxygen  be  4.78,  the  total 
oxygen  absorbed  may  be  obtained  by  the  rule  of  simple  proportion,  e.g.: 

100  :  4.78  :  :  8,500  :  x  =  406  liters  or  580  grams* 
Or 

100  :  4.78  :  :  12,752  :  x  =  609  liters  or  870  grams. 

By  the  same  method  the  total  carbon  dioxid  exhaled  is  found  to  be  either 
372  liters  or  73.5  grams,  or  558  liters  or  1,103  grams,  volumes  in  both  in- 
stances which  agree  very  well  with  volumes  obtained  by  other  methods. 

As  there  is  always  more  oxygen  consumed  than  carbonic  acid  exhaled, 
and  as  oxygen  unites  with  carbon  to  form  an  equal  volume  of  carbonic  acid, 
it  is  evident  that  a  certain  quantity  of  oxygen  disappears  within  the  body. 
In  all  probability  it  unites  with  the  surplus  hydrogen  of  the  food  to  form 
water. 

The  quantities  both  of  oxygen  absorbed  and  carbon-dioxid  exhaled 
daily  is  subject  to  considerable  variation.  They  are  increased  by  exercise, 
digestion  and  a  lowered  temperature,  and  decreased  by  the  opposite 
conditions. 

The  gain  in  watery  vapor  will  depend  on  the  amount  previously  present 
in  the  air.  This  is  conditioned  by  the  temperature.  With  a  rise  in  tem- 
perature the  percentage  of  water  increases;  with  a  fall,  it  decreases. 

*  I  liter  of  oxygen  weighs  1.4298  grams;  i  liter  of  carbon  dioxid  weighs  1.977  grams. 


132  HUMAN  PHYSIOLOGY 

The  gain  in  organic  matter  is  also  variable.  The  amount  present  is  not 
sufficient  to  permit  of  a  thorough  chemic  analysis,  but  there  are  reasons  for 
believing  that  it  belongs  to  the  protein  group  of  bodies.  If  it  accumulates 
in  the  air,  especially  at  high  temperatures,  it  readily  undergoes  decomposi- 
tion, with  the  production  of  offensive  odors.  Traces  of  free  ammonia  have 
also  been  found  in  the  expired  air.  In  addition  to  these  chemic  changes, 
the  air  experiences  physical  changes;  e.g.,  a  rise  in  temperature  and  an  in- 
crease in  volume.  The  rise  in  temperature  can  be  shown  by  breathing 
through  a  suitable  mouthpiece  into  a  glass  tube  containing  a  thermometer. 
By  this  means  it  has  been  shown  that  inspired  air  at  20°C.  rises  in  tempera- 
ture to  37°C.;  at  6.3°  to  29.8°C.  The  increase  in  the  temperature  will 
depend  upon  that  of  the  air  inspired  and  the  time  it  remains  in  the  lungs. 
If  retained  a  sufficient  length  of  time  it  will  always  become  that  of  the 
body. 

Changes  in  the  Composition  of  the  Blood. — As  the  blood  of  the  pulmonic 
artery  passes  through  the  pulmonic  capillaries,  it  loses  carbon  dioxid  and 
gains  oxygen,  in  consequence  of  which,  it  changes  in  color  from  a  bluish  red 
to  a  scarlet  red.  As  the  blood  of  the  systemic  arteries  flows  into  and 
through  the  systemic  capillaries  it  loses  oxygen  and  gains  carbon  dioxid 
in  consequence  of  which  it  changes  in  color  from  a  scarlet  red  to  a  bluish 
red. 

The  Gases  of  the  Blood. — The  presence  of  gases  in  the  blood  is  demon- 
strated by  subjecting  it  to  the  vacuum  of  the  air  pump  into  which  they  at 
once  escape. 

An  analysis  of  the  gases  so  obtained  gives  the  following  results. 

f  Oxygen 20  vols.  f  Oxygen 12  vols. 

Arterial  blood  \  Carbon  dioxid.    ^o  vols.  Venous  blood  \  Carbon  dioxid.     45  vols. 
100  vols.        [  Nitrogen 1-2  vols.       100  vols.        [  Nitrogen 1-2  vols. 

The  changes  produced  in  the  blood  by  respiration,  both  external  and  inter- 
nal, become  apparent  from  a  comparison  of  these  analyses.  The  arterial 
blood  while  passing  through  the  capillaries  of  the  tissues  loses  eight  vol- 
umes per  cent,  of  oxygen  and  gains  five  per  cent,  of  carbon  dioxid.  The 
venous  blood  while  passing  through  the  capillaries  of  the  lungs  gains 
oxygen  and  loses  carbon  dioxid  in  corresponding  amounts.  These  amounts 
will  vary  somewhat  in  the  analyses  of  the  blood  of  different  animals  and 
under  different  physiologic  conditions.  The  volume  of  nitrogen  is  not 
appreciably  changed. 

The  Condition  of  the  Gases  in  the  Blood. — After  the  oxygen  of  the  alve- 
oli passes  across  the  thin  alveolo-capillary  wall  into  the  blood  it  combines 
with  hemoglobin  to  form  oxy-hemoglobin,  the  compound  that  gives  the 


RESPIRATION  1 33 

scarlet-red  color  to  the  arterial  blood.  As  the  arterial  blood  flows  into 
the  capillary  vessels  the  oxygen  is  in  part  dissociated  from  the  hemoglobin 
and  passes  across  the  capillary  wall  into  the  tissues. 

The  carbon  dioxid  arising  in  the  tissues  passes  across  the  capillary  wall 
into  the  blood  where  a  portion  of  it  is  physically  absorbed,  while  another 
portion  combines  with  sodium  carbonate  to  form  a  bicarbonate.  As  the 
blood  passes  through  the  pulmonic  capillaries  the  carbon  dioxid  is  in  part 
dissociated  and  then  passes  across  the  alveolo-capillary  wall  into  the 
interior  of  the  alveoli. 

Changes  in  the  Composition  of  the  Tissue  Fluids. — An  analysis  of  the 
tissue  fluids  shows  the  absence  of  oxygen  and  the  presence  of  carbon  dioxid. 
Notwithstanding  the  continuous  passage  of  oxygen  across  the  capillary 
walls  into  the  tissue  fluids  free  oxygen  cannot  be  determined  in  them. 
The  absence  of  oxygen  would  indicate  that  it  is  immediately  utilized  by 
the  tissue  with  the  production  of  carbon  dioxid;  or  that  it  is  stored  in  the 
tissues  in  some  form  or  other  by  which  it  can  be  retained  until  required  for 
oxidation  purposes — the  latter  is  the  more  likely  view. 

The  carbon  dioxid  is  present  in  variable  quantities  in  the  tissues  and 
fluids  and  though  passing  into  the  blood  at  varying  rates  it  is  as  constantly 
being  evolved. 

The  Mechanism  of  the  Gaseous  Exchange. — The  passage  of  the  oxygen 
from  the  alveoli  into  the  blood  and  into  the  tissues,  and  the  passage  of  the 
carbon  dioxid  from  the  tissues  into  the  blood  and  into  the  alveoli  is  believed 
to  be  due  to  differences  of  pressure.  In  the  alveoli  the  oxygen  pressure  is 
approximately  equal  to  130  mm.  of  Hg.;  in  the  arteries  106  mm.  Hg.  and 
in  the  tissues  zero.  In  the  tissues  the  carbon-dioxid  pressure  varies  from 
45  to  68  mm.  of  Hg.;  in  the  veins  42  mm.  Hg.  and  in  the  alveoli  about  38 
mm.  Hg.  In  these  differences  of  pressure  is  to  be  found  an  explanation  for 
the  exchange  of  these  gases. 

The  Total  Respiratory  Exchange. — The  total  quantities  of  oxygen  ab- 
sorbed and  carbon  dioxid  discharged  by  a  human  being  in  twenty-four 
hours  are  measures  of  the  intensity  of  the  respiratory  process,  and  an  indi- 
cation of  the  extent  and  character  of  the  chemic  changes  attending  all  life 
phenomena. 

Approximate  amounts  of  oxygen  absorbed  and  carbon  dioxid  exhaled 
as  determined  by  different  investigators  are  as  follows: 

Oxygen  absorbed  Observer  Carbon  dioxid  discharged 

746  grams.  Vierordt.  876  grams. 

700  grams.  Pettenkofer  and  Voit.  800  grams. 

663  grams.  Sx)€ck.  770  grams. 


134  HUMAN  PHYSIOLOGY 

The  amounts  of  oxygen  absorbed  in  Pettenkofer  and  Voit's  experi- 
ments varied  from  594  to  1,072  grams;  of  carbon  dioxid  exhaled,  from 
686  to  1,285  grams. 

The  Nerve  Mechanism  of  Respiration.— The  simultaneous  and  coordi- 
nated activity  of  the  inspiratory  muscles  implies  the  simultaneous  and 
coordinated  activity  of  nerve  centers  and  their  related  motor  nerves. 
Thus  the  action  of  the  nasal  and  laryngeal  muscles  (the  dilatator  naris  and 
the  posterior  crico-arytenoid)  involves  the  activity  of  the  facial  and  infe- 
rior laryngeal  nerves  respectively,  the  centers  of  origin  of  which  lie  in  the 
gray  matter  beneath  the  floor  of  the  fourth  ventricle;  the  diaphragm  and 
intercostal  muscles  involve  respectively  the  activity  of  the  phrenic  and 
intercostal  nerves,  the  centers  of  origin  of  which  lie  in  the  anterior  horn  of 
the  gray  matter  of  the  spinal  cord  at  a  level,  for  the  phrenic,  of  the  fourth, 
fifth,  and  sixth  cervical  nerves,  and  for  the  intercostals  at  the  level  of  the 
thoracic  nerves.  Division  of  any  one  of  these  nerves  is  followed  by 
paralysis  of  its  related  muscle. 

Inspiratory  Center. — The  coordinate  contraction  of  the  inspiratory 
muscles  implies  a  practically  simultaneous  discharge  of  nerve  impulses 
from  each  of  the  foregoing  nerve-centers,  accurately  graduated  in  intensity 
in  accordance  with  inspiratory  needs.  This  has  been  supposed  to  necessi- 
tate the  existence  in  the  central  nerve  system  of  a  single  group  of  nerve- 
cells  from  which  nerve  impulses  are  rhythmically  discharged  and  conducted 
to  the  previously  mentioned  nerve-centers  in  the  medulla  oblongata 
and  spinal  cord,  by  which  they  are  in  turn  excited  to  activity.  To  this 
group  of  cells  the  term  "inspiratory  center"  has  been  given. 

The  rhythmic  activity  of  the  inspiratory  center  is  in  part  the  result  of  the 
stimulating  action  of  carbon  dioxid  and  partly  the  result  of  the  trans- 
mission to  it  of  nerve  impulses  from  various  regions  of  the  body.  The 
irritability  of  the  center  is  markedly  increased  by  the  percentage  of  carbon 
dioxid  in  the  blood  and  decreased  by  the  opposite  condition.  The  vagus 
nerves  of  all  afferent  nerves  are  the  most  influential  in  maintaining  the 
normal  rhythmic  discharge  of  nerve  impulses  from  the  inspiratory  center 
as  shown  by  the  effects  that  follow  their  separation  from  the  center. 
Thus,  if  while  the  animal  is  breathing  regularly  and  quietly  both  vagi  are 
cut,  the  respiratory  movements  become  much  slower,  falling  perhaps  to 
one-third  their  original  number  per  minute.  At  the  same  time  the  in- 
spirations become  deeper  and  somewhat  spasmodic  in  character.  The 
duration  of  the  inspiratory  movement  is  also  increased  beyond  that  of  the 
expiratory  movement.  If  now  the  central  end  of  one  of  the  divided  vagi 
be  stimulated  with  weak  induced  electric  currents,  the  respiratory  move- 


RESPIRATION  1 35 

ments  are  again  increased  in  frequency,  and  their  depth  diminished  until 
the  normal  rate  is  restored.  With  the  cessation  of  the  stimulation  the 
former  condition  at  once  returns.  This  would  seem  to  indicate  that  the 
vagus  nerve  contains  nerve-fibers  which,  under  physiologic  conditions, 
transmit  nerve  impulses  which  inhibit  the  inspiratory  discharge  and  lead 
to  an  expiratory  movement  sooner  than  would  otherwise  be  the  case,  and 
thus  maintain  the  normal  rate  and  extent  of  the  inspiratory  charge. 

Stimulation  of  the  central  end  of  the  divided  vagus  with  strong  electric 
currents  excites  the  activity  of  the  inspiratory  center  to  such  an  extent, 
that  the  muscles  pass  into  the  tetanic  state  and  the  thorax  comes  to  rest  in 
the  condition  of  a  forced  inspiration. 

These  results  indicate  that  the  vagus  nerve  contains  two  classes  of 
fibers  which  influence  the  activity  of  the  inspiratory  center,  viz. :  an  exci- 
tator  and  an  inhibitor.  The  stimulus  to  their  excitation  is  to  be  found  in 
the  alternate  recoil  and  expansion  of  the  alveoli,  in  the  walls  of  which  they 
terminate.  With  the  recoil  of  the  alveolar  walls  nerve  impulses  are 
developed  which  ascend  the  vagi  to  the  inspiratory  center  and  excite  it  to 
activity  and  thus  call  forth  a  new  inspiratory  movement  sooner  than  it 
would  otherwise  take  place.  With  the  expansion  of  the  alveoli,  nerve 
impulses  are  developed  which  ascend  the  vagi  to  the  inspiratory  center  and 
inhibit  its  activity  and  thus  lead  to  an  expiratory  movement  sooner  than  it 
would  otherwise  take  place.  The  respiratory  mechanism  is  apparently 
self-regulative  and  maintained  by  the  alternate  recoil  and  expansion  of  the 
lungs. 

The  Establishment  of  Respiration  after  Birth. — During  intra-uterine 
life  the  exchange  of  gases  is  accomplished  by  the  placenta.  Immediately 
after  birth,  this  method  is  abolished.  The  cause  of  the  first  inspiration 
therefore  must  be  associated  with  an  increase  in  the  percentage  of  carbon 
dioxid  or  a  decrease  in  the  percentage  of  oxygen  in  the  blood.  The  former 
condition  is  more  likely  to  be  the  efficient  cause.  The  rapid  accumulation 
of  carbon  dioxid  with  its  increasing  pressure  in  the  inspiratory  center  so 
raises  its  irritability,  as  to  lead  to  a  discharge  of  nerve  impulses  which  are 
conducted  to  the  inspiratory  muscles  and  cause  their  contraction.  With 
the  first  inspiration  thus  established  the  nerve  mechanism  comes  into 
play. 

Inasmuch  as  cold  water  applied  to  the  skin  of  the  adult  profoundly 
excites  at  times  the  inspiratory  center  it  has  been  assumed  that  an  addi- 
tional factor  leading  to  an  excitation  of  the  inspiratory  center  is  the  rapid 
cooling  of  the  surface  of  the  child  by  the  evaporation  of  the  amniotic  fluid 
from  the  surface  of  the  skin.  The  nerve  impulses  thus  developed  are 
transmitted  through  cutaneous  nerves  to  the  inspiratory  center.    This 


136  HUMAN   PHYSIOLOGY 

assumption  is  somewhat  strengthened  by  the  fact  that  in  delayed  inspi- 
ration the  stimulation  of  the  skin  by  the  application  of  cold  water 
frequently  leads  to  a  sudden  inspiratory  movement. 

ANIMAL  HEAT 

■  The  animal  body  possesses  a  temperature  that  is  perceptible  to  the  sense 
of  touch  and  determinable  by  a  thermometer.  This  temperature  is  the 
result  of  the  liberation  of  heat  which  attends  the  chemic  changes  taking 
place  in  the  tissues  and  organs  of  the  living  body  and  which  underlie  all 
manifestations  of  life.  In  consequence  of  this  each  animal  acquires  a 
certain  body-temperature. 

The  normal  temperature  of  the  body  in  the  adult,  as  shown  by  means  of 
a  delicate  thermometer  placed  in  the  axila,  ranges  from  97.25°F.  to 
99.5°F.,  though  the  mean  normal  temperature  is  estimated  by  Wunderlich 
at  98.6°F. 

The  temperature  varies  in  different  portions  of  the  body  however, 
according  to  the  extent  to  which  oxidation  rakes  place,  being  highest  in  the 
muscles,  in  the  brain,  blood,  liver,  etc. 

Variations  in  the  Mean  Temperature. — The  conditions  which  produce 
variations  in  the  normal  temperature  of  the  body  are:  age,  period  of  the 
day,  exercise,  food  and  drink,  climate,  season,  and  disease. 

Age. — At  birth  the  temperature  of  the  infant  is  about  i°F.  above  that  of 
the  adult,  but  in  a  few  hours  falls  to  95.5°F.,  to  be  followed  in  the  course 
of  twenty-four  hours  by  a  rise  to  the  normal  or  a  degree  beyond.  During 
childhood  the  temperature  approaches  that  of  the  adult;  in  aged  persons 
the  temperature  remains  about  the  same,  though  they  are  not  so  capable 
of  resisting  the  depressing  effects  of  external  cold  as  adults.  A  diurnal 
variation  of  the  temperature  occurs  from  i.8°F.  to  3.7°F.  (Jiirgensen); 
the  maximum  occurring  late  in  the  afternoon,  from  4  to  9  p.  m.;  the 
minimum,  early  in  the  morning,  from  i  to  7  a.  m. 

Exercise. — The  temperature  is  raised  from  i**  to  2°F.  during  active  con- 
tractions of  the  muscular  masses,  and  is  probably  due  to  the  increased 
activity  of  chemic  changes;  arise  beyond  this  point  being  prevented  by  its 
diffusion  to  the  surface,  consequent  on  a  more  rapid  circulation,  radiation, 
more  rapid  breathing,  etc. 

Food  and  Drink. — The  ingestion  of  a  hearty  meal  increases  the  tempera- 
ture but  slightly;  an  absence  of  food,  as  in  starvation,  produces  a  marked 
decrease.  Alcoholic  drinks,  in  large  amounts,  in  persons  unaccustomed  to 
their  use,  cause  a  depression  of  the  temperature  amounting  to  from  i"^  to 
2°F.    Tea  causes  a  slight  elevation. 


ANIMAL  HEAT  1 37 

External  Temperature. — Long-continued  exposure  to  cold,  especially  if 
the  body  is  at  rest,  diminishes  the  temperature  from  i°  to  2°F.,  while  ex- 
posure to  a  great  heat  slightly  increases  it. 

Disease  frequently  causes  a  marked  variation  in  the  normal  temperature 
of  the  body,  which  rises  as  high  as  io7°F.  in  typhoid  fever  and  io5°F.  in 
pneumonia;  in  cholera  it  falls  as  low  as  80° F.  Death  usually  occurs  when 
the  heat  remains  high  and  persistent,  from  106°  to  iio°F.;  the  increase  of. 
heat  in  disease  is  due  to  excessive  production  rather  than  to  diminished 
elimination. 

The  Residual  Heat  of  the  Body. — As  a  preliminary  to  a  consideration  of 
heat-production  and  heat-dissipation,  it  is  of  interest  to  determine  the 
actual  quantity  of  heat  expressed  in  Calories,  that  resides  in  the  body  at  all 
times.  This  can  be  approximately  determined  from  the  chemic  composi- 
tion and  the  temperature.  A  chemic  analysis  of  the  body  shows  that  it 
consists  of  water  0.6,  and  of  tissue  0.4.  If  the  weight  be  assumed  to  be  70 
kilograms  then  42  kilograms  consist  of  water,  and  as  the  temperature  is 
3 7° C,  the  42  kilos  of  water  will  contain  42  X  37  or  1,5 54  kilogram  calories; 
the  remaining  28  kilograms  consist  of  tissues,  the  specific  heat  of  which  is 
but  0.8  that  of  water,  hence  the  28  kilograms  of  tissue  will  contain  28  X 
0.8  calories;  the  equivalent  of  22.4  kilograms  of  water.  Since  the  tem- 
perature of  the  body  is  37°C.  the  additional  number  of  Calories  will 
be  22.4  X  37  or  828,  making  a  total  of  2,382  Calories  an  amount  of  heat 
absolutely  necessary  to  maintain  the  body- temperature  at  the  physio- 
logical level.  Notwithstanding  the  constant  liberation  of  large  amounts 
of  heat  each  day,  it  is  dissipated  more  or  less  rapidly  in  accordance  with 
variations  in  temperature,  character  of  clothing  and  a  variety  of  other 
conditions,  and  so  accurately  is  this  done,  that  at  the  end  of  the  twenty- 
four  hours  the  body  possesses  its  customary  quantity  of  heat  and  its 
physiologic  temperature. 

Heat  Production.  Thermogenesis. — The  immediate  source  of  the  body 
heat  is  to  be  found  in  the  chemic  changes  that  take  place  in  all  the  tissues 
and  organs  of  the  body. 

Every  contraction  of  a  muscle,  every  act  of  secretion,  each  exhibition  of 
nerve  force,  is  accompanied  by  a  change  in  the  chemic  composition  of  the 
tissues  and  an  evolution  of  heat.  The  reduction  of  the  disintegrated  tis- 
sues to  their  simplest-form  by  oxidation,  and  the  combination  of  the  oxygen 
of  the  inspired  air  with  the  carbon  and  hydrogen  of  the  blood  and  tissues, 
results  in  the  formation  of  carbonic  acid  and  water  and  the  liberation  of  a 
great  amount  of  heat. 


138  HUMAN  PHYSIOLOGY 

Certain  elements  of  the  food,  particularly  the  carho-hydratej  and  tYi&fatSy 
undergo  oxidation  without  taking  part  in  the  formation  of  the  tissues, 
being  transformed  into  carbon  dioxid  and  water,  and  thus  increase  the 
sum  of  heat  in  the  body. 

The  total  quantity  of  heat  liberated  each  day  may  be  approximately 
determined  in  at  least  two  ways:  (i)  by  determining  experimentally  the 
•  heat  values  of  different  food  principles  by  direct  oxidation;  (2)  by  collect- 
ing and  measuring  with  a  suitable  apparatus,  a  calorimeter,  the  heat 
evolved  by  the  oxidation  of  the  food  within,  and  dissipated  from,  the 
body  daily. 

By  the  direct  oxidation  of  the  food  principles  by  means  of  a  calorimeter, 
it  has  been  determined,  when  they  are  burned  to  carbon  dioxid  and  water, 
that  I  gram  of  protein  yields  approximately  5.6  Calories,  i  gram  of  fat 
9-353  C.  and  i  gram  of  starch  or  sugar  4.1 16  C.  In  the  body  fat  and  sugar 
or  starch  are  also  burned  to  carbon  dioxid  and  water.  The  protein,  how- 
ever, is  only  in  part  burned  to  this  extent;  a  part  is  changed  to  urea  which 
when  eliminated  carries  with  it  a  portion  of  the  original  heat  of  the  protein. 
In  the  body  the  protein  yields  4.124  Calories.  The  total  number  of  calor- 
ies liberated  by  the  various  diet  scales  (see  page  53)  can  be  readily 
determined  by  multiplying  the  quantities  of  the  food  principles  by  the  fore- 
going factors.     The  diet  scale  of  Vieordt,  for  example,  yields  the  following: 

120  grams  of  protein 494  •  88  Calories 

90  grams  of  fat 841 .  77  Calories 

330  grams  of  starch i,358 .  28  Calories 

Total 2,694 •  93  Calories 

The  total  calories  obtained  from  other  diet  scales  would  be  as  follows: 
Ranke's,  2,335;  Voit's,  3,387;  Moleschott's,  2,984;  Atwater's,  3,331- 
These  numbers  indicate  theoretically  the  total  heat-production  in  the 
body  daily. 

The  collection  of  the  heat  dissipated  by  a  human  being  weighing  70 
kilos  when  placed  in  a  suitable  calorimeter  reveals  the  fact  that  it  amounts 
to  from  2,300  to  3,000  Calories. 

The  amount  of  heat  liberated  will  naturally  vary  in  accordance  with  a 
number  of  conditions  but  principally  with  variations  as  physiologic  activity, 
the  quantity  and  quality  of  food  and  changes  in  the  external  temperature. 
The  chief  factor  that  increases  metabolism  and  hence  heat  produc- 
tion is  a  low  external  temperature.  This  in  turn  leads  to  increased 
physical  activity  and  increased  food  consumption.  The  heat-production 
and  elimination  under  such  circumstances  may  reach  4,700  Calories  a  day. 


ANIMAL  HEAT  1 39 

Heat  Dissipation.  Thermolysis. — From  the  preceding  statements  it  is 
evident  that  the  body  is  continually  liberating  heat  in  amounts  daily  far  in 
excess  of  that  necessary  for  the  maintenance  of  the  body-temperature. 
Should  this  heat  be  retained,  the  temperature  of  the  body  would  be  raised 
at  the  end  of  twenty-four  hours  an  additional  18°  or  2o°C. — a  temperature 
far  in  excess  of  that  compatible  with  the  maintenance  of  physiologic  proc- 
esses. That  the  body  may  be  kept  at  the  mean  temperature  of  37°C.  it  is 
essential  that  the  heat  liberated  be  dissipated  as  fast  as  it  is  produced,  or  to 
state  the  problem  in  another  way,  the  heat  dissipated  by  the  body  must  be 
replaced  by  an  equal  amount  liberated,  if  equilibrium  of  temperature  is  to 
be  maintained.  The  dissipation  of  the  heat  is  accomplished  in  several 
ways : 

Assuming  2,500  Calories  to  be  an  average  of  heat  liberated  during  a 
day  of  repose,  the  losses,  in  the  ways  stated  in  the  foregoing  paragraph,  may 
be  tabulated  as  follows: 

1.  In  Warming  Food  and  Drink. — The  average  temperature  of  food  and 
drink  is  about  i2°C.;  the  amount  of  both  together  is  about  3  kilograms; 
the  specific  heat  of  food  about  0.8  that  of  water.  The  absorption  of  body- 
heat,  therefore,  by  the  food  amounts  approximately  to  3  X  0.8  X  25°C.  = 
60  Calories  =  2.8  per  cent.  With  the  removal  of  the  end-products  of  the 
foods  and  drink  from  the  body  an  equal  amount  of  heat  is  carried  out. 

2.  In  Warming  the  Inspired  Air. — The  average  temperature  of  the  air 
is  i2°C.;  the  amount  of  inspired  air,  about  15  kilograms;  the  specific 
heat  of  air,  0.26.  The  absorption  of  body-heat  by  the  air  until  it  attains 
the  temperature  of  the  body  will,  therefore,  amount  to  15  X  0.26  X  25°  = 
97.5  Calories  =  3.8  per  cent.  The  expired  air  removes  from  the  body  a 
corresponding  amount. 

3.  In  the  Evaporation  of  Water  from  the  Lungs. — The  quantity  of  water 
evaporated  from  the  lungs  may  be  estimated  at  400  grams;  as  each 
gram  requires  for  its  evaporation  0.582  Calorie,  the  quantity  of  heat 
lost  by  this  channel  would  be  400  X  0.582  =  232.8  Calories  =  9.4  per 
cent. 

4.  In  the  Evaporation  oj  Water  from  the  Skin. — The  quantity  of  water 
evaporated  from  the  skin  may  be  estimated  at  660  grams,  causing  a  loss  of 
heat  by  this  channel  of  660  X  0.582  =  384.1  Calories  =15.3  per  cent. 

5.  In  Radiation  and  Conduction  from  the  Skin. — The  amount  of  heat  lost 
by  this  process  can  be  indirectly  determined  only  by  subtracting  the  total 
amount  lost  by  the  above-mentioned  channels  from  the  total  amount  pro- 
duced. Thus,  2,500  —  7,774.4  =  1,725.6  Calories  =  69  per  cent,  would 
represent  the  average  amount  lost  by  radiation  and  conduction. 


140  HUMAN   PHYSIOLOGY 

Head  dissipation  is  accomplished  as  shown  in  the  foregoing  tabulation 
mainly  by  radiation  and  conduction,  70  per  cent.,  and  by  the  evaporation 
of  water  from  the  lungs  and  skin,  25  per  cent.  The  mechanism  by  which 
this  dissipation  is  accomplished  consists  of  the  cutaneous  and  pulmonic 
blood-vessels  and  the  sweat-glands  which  may  be,  therefore,  regarded  as 
thermolytic  organs.  The  ratio  of  the  heat  loss  between  the  evaporation 
of  water  and  radiation  will  vary  with  the  temperature,  the  season  of  the 
year,  the  character  of  the  clothing,  etc. 

Inasmuch  as  the  mean  temperature  of  the  body  remains  practically  con- 
stant, notwithstanding  seasonal  variations,  it  is  apparent  that  heat-dissi- 
pation must  be  exactly  balanced  by  heat-production.  Should  there  be 
any  want  of  correspondence  between  the  two  processes,  there  would  arise 
either  an  increase  or  a  decrease  in  the  mean  temperature.  As  both  heat- 
production  and  heat-dissipation  are  variable  factors,  dependent  on  a  vari- 
ety of  internal  and  external  conditions,  their  adjustment  is  accomplished 
by  a  complex  self-regulating  mechanism  involving  muscle,  vascular,  and 
secretor  elements,  coordinated  by  the  nerve  system. 

EXCRETION 

Excretion  may  be  defined  as  the  process  by  which  the  end-products  of 
metabolism  are  removed  from  the  body.  As  the  retention  of  these  end- 
products  in  the  body  would  exert  a  deleterious  influence  on  normal  met- 
abolism, their  prompt  removal  becomes  essential  to  the  maintenance  of 
physiologic  activity.  The  principal  excretions  of  the  body — urine,  per- 
spiration, and  bile — are,  with  the  exception  of  those  given  off  in  the 
lungs,  complex  fluids  in  which  are  to  be  found  in  varying  proportions  the 
chief  end-products  of  metabolism. 

The  chief  excretory  organs,  therefore,  are  the  kidneys,  skin  and  lungs. 

URINE 

Normal  tirine  is  of  a  pale  yellow  or  amber  color,  perfectly  transparent, 
with  an  aromatic  odor,  an  acid  reaction,  a  specific  gravity  of  1,020,  and  a 
temperature  when  first  discharged  of  ioo*'F. 

The  color  varies  considerably  in  health,  from  a  pale  yellow  to  a  brown 
hue,  owing  to  the  presence  of  the  coloring-matter,  urobilin  or  urochrome. 

The  transparency  is  diminished  by  the  presence  of  mucus,  the  calcium 
and  magnesium  phosphates,  and  the  mixed  urates. 

The  reaction  of  the  urine  is  acid,  owing  to  the  presence  of  acid  phosphate 
of  sodium.     The  degree  of  acidity,  however,  varies  at  different  periods  of 


URINE  141 

the  day.  Urine  passed  in  the  morning  is  strongly  acid,  while  that  passed 
during  and  after  digestion,  especially  if  the  food  is  largely  vegetable  in 
character,  is  either  neutral  or  alkaline. 

The  specific  gravity  varies  from  1,015,  to  1,025. 

The  quantity  of  urine  excreted  in  twenty-four  hours  is  between  forty  and 
fifty  fluidounces,  but  ranges  above  and  below  this  standard. 

The  odor  I?,  characteristic,  and  caused  by  the  presence  of  taurylic  and 
phenylic  acids,  but  is  influenced  by  vegetable  foods  and  other  substances 
eliminated  by  the  kidneys. 

The  chemic  composition  of  the  urine  is  very  complex  and  is  determined 
partly  by  the  metabolism  of  the  constituents  of  the  tissues  and  partly  by 
the  quantity  and  the  quality  of  the  food  consumed  and  metabolized. 
Hence  the  composition  will  vary  from  day  to  day  in  accordance  with  the 
character  of  the  food.  An  average  composition  is  presented  in  the 
following  table: 

The  Chemical  Composition  of  Urine 

Water   iSoo .  00  c.c. 

Total  solids 72  .  00  grams. 

Urea 33  iS  grams. 

Uric  acid  (urates) o .  55  grams. 

Hippuric  acid  (hippurates)    o .  40  grams. 

Kreatinin,  xanthin,  hypoxanthin,  guanin,  am-  1 


,^       .          ^      .  ,  XX  .21  grams, 

monium  salts,  pigment,  etc J 

Inorganic  salts;  sodium  and  potassium  sulphates, 
phosphates,  and  chlorids;  magnesium  and 
calcium  phosphates |-27.00  grams. 

Organic  salts:  lactates,  acetates,  formates  . 
in    small    amounts      

Sugar a  trace. 

Gases,  nitrogen,  and  carbonic  acid. 

The  Total  Solids. — It  is  frequently  a  matter  of  clinic  interest  to  deter- 
mine the  total  amount  of  the  solid  constituents  excreted  in  twenty-four 
hours.  This  may  be  attained  approximately  by  multiplying  the  last  two 
figures  of  the  specific  gravity  by  the  coefficient,  2.33,  of  Haeser  or 
Christison.  The  coefficient  of  Jones,  2.6,  is  believed  by  some  observers  to 
give  more  accurate  results  for  conditions  existing  in  this  country.  The 
result  expresses  the  total  solids  in  1,000  parts:  e.g.,  urine  with  a  specific 
gravity  of  1.020  would  contain  20  X  2.33,  or  46.60  grams  of  solid  matter 
per  1,000  c.c.  If  the  amount  passed  in  twenty-four  hours  be  1,500  c.c, 
the  total  solids  would  amount  to  69.9  grams  daily. 

Organic  Constituents  of  Urine. — Urea  is  one  of  the  most  important  of 
the  organic  constituents  of  the  urine,  and  is  present  to  the  extent  of  from  2.5 


142  HUMAN  PHYSIOLOGY 

to  3.2  per  cent.  Urea  is  a  colorless,  neutral  substance,  crystallizing  in 
four-sided  prisms  terminated  by  oblique  surfaces.  When  crystallization  is 
caused  to  take  place  rapidly,  the  crystals  take  the  form  of  long,  silky 
needles.  Urea  is  soluble  in  water  and  alcohol;  when  subjected  to  pro- 
longed boiling,  it  is  decomposed,  giving  rise  to  carbonate  of  ammonia. 
In  the  alkaline  fermentation  of  urine,  urea  takes  up  two  molecules  of 
water  with  the  production  of  carbonate  of  ammonia. 

The  average  amount  of  urea  excreted  daily  has  been  estimated  at  about 
34  grams.  As  urea  is  one  of  the  principal  products  of  the  breaking  up  of 
the  protein  compounds  within  the  body,  it  is  quite  evident  that  the  quan- 
tity produced  and  eliminated  in  twenty-four  hours  will  be  increased  by 
any  increase  in  the  amount  of  protein  food  consumed,  or  by  a  rapid 
destruction  of  protein  tissues,  as  is  observed  in  various  pathologic  states, 
inanition,  febrile  conditions,  fevers,  etc.  A  farinaceous  or  vegetable  diet 
will  diminish  the  urea  production  nearly  one-half. 

Muscular  exercise  when  the  nutrition  of  the  body  is  in  a  state  of 
equilibrium  does  not  seem  to  increase  the  quantity  of  urea. 

Seat  of  Formation  and  Antecedents  of  Urea. — As  to  the  seat  of  urea 
formation  there  is  some  discussion.  It  is  quite  certain  that  urea  pre-exists 
in  the  blood  and  is  merely  excreted  by  the  kidneys.  Experimental  and 
pathologic  facts  point  to  the  liver  as  the  probable  organ  engaged  in  urea 
formation.  Acute  yellow  atrophy  of  the  liver,  suppurative  diseases  of  the 
liver,  diminish  almost  entirely  the  production  of  urea,  but  increase  the 
amount  of  the  ammonium  salts  in  the  urine.  The  perfusion  of  the  liver 
of  a  recently  killed  animal  with  a  given  amount  of  blood  containing 
ammonium  salts  will  be  followed  after  the  lapse  of  several  hours  by  an 
amount  of  urea  in  the  blood  two  or  three  times  the  normal  quantity. 
These  and  other  facts  indicate  that  the  chief  seat  of  urea  formation  is  to  be 
found  in  the  liver  cells. 

The  antecedents  of  urea,  out  of  which  the  hepatic  cells  construct  urea 
have,  for  chemic  reasons  as  well  as  from  the  foregoing  experimental 
results,  been  shown  to  be  the  salts  of  ammonia,  the  carbonate,  carbamate, 
and  lactate.  The  source  of  the  ammonia  is  probably  in  part  the  intestine, 
as  this  compound  is  one  of  the  products  of  the  hydrolysis  and  cleavage  of 
the  proteins  during  digestion.  That  this  is  the  case  is  apparent  from  the 
fact  that  the  blood  of  the  portal  vein  always  contains  more  ammonia  than 
the  blood  of  any  other  region  of  the  vascular  apparatus. 

It  has  also  been  established  that  of  the  amino-acids  circulating  in  the 
blood  and  tissues  a  certain  number  not  needed  for  growth  and  tissue 
repair,  undergo  a  cleavage  into  an  NH2  portion  and  a  carbonaceous 


URINE  143 

radicle.  The  former  is  then  converted  into  ammonia  and  subsequently 
into  urea  by  the  liver  cells  or  perhaps  the  muscles  and  other  tissues  as  well. 
The  protein  constituents  of  the  tissues  may  in  their  katabolism  likewise 
yield  the  NH2  element,  which  is  also  subsequently  transformed  into 
ammonia  and  urea. 

Uric  acid  is  also  a  constant  ingredient  of  the  urine  and  is  closely  allied  to 
urea.  It  is  a  nitrogen-holding  compound,  carrying  out  of  the  body  a  por- 
tion of  the  nitrogen.  The  amount  eliminated  daily  varies  from  0.5  to  i 
gram.  Uric  acid  is  a  colorless  crystal  belonging  to  the  rhombic  system. 
It  is  insoluble  in  water,  and  if  eliminated  in  excessive  amounts,  it  is  de- 
posited as  a  "brick-red"  sediment  in  the  urine.  It  is  doubtful  if  uric  acid 
exists  in  a' free  state,  being  combined  for  the  most  part  with  sodium  and 
potassium  bases  forming  urates.  It  is  to  be  regarded  as  one  of  the  terminal 
products  of  the  decomposition  of  nucleic  acid  which  in  turn  is  derived  from 
nuclein,  a  constituent  of  cell  nuclei. 

Hippuric  acid  is  found  very  generally  in  urine,  though  it  is  present  only 
in  small  amounts.  It  is  increased  by  a  diet  as  asparagus,  cranberries, 
plums,  and  by  the  administration  of  benzoic  and  cinnamic  acids.  It  is 
probably  formed  in  the  kidney. 

Kreatinin. — This  is  a  crystalline  nitrogenous  compound  closely  resem- 
bling kreatin,  one  of  the  constituents  of  muscle-tissue.  The  amount  ex- 
creted daily  is  about  i  gram.  The  origin  of  kreatinin  is  not  very  clear. 
It  is  probably,  however,  that  if  kreatin  is  capable  of  transformation  into 
kreatinin  a  certain  portion  is  derived  from  the  kreatin  contained  in  the 
meat  consumed  as  food.  But  as  kreatinin  is  steadily  excreted  though  in 
less  amounts  on  a  diet  from  which  meat  is  excluded  it  is  certain  that  this 
portion  at  least  must  have  some  other  source  containing  nitrogen,  and  the 
inference  is  that  it  is  one  of  the  end-products  of  the  protein  metabolism 
that  is  taking  places  in  tissues  generally  and  more  particularly  in  muscle- 
tissue. 

Xanthin,  Hypoxanthin,  Adenin,  Guanin. — These  compounds  are  also 
found  in  urine  in  small  but  variable  amounts.  They  are  nitrogenized 
compounds  derived  mainly  from  the  metabolism  of  the  nuclein  bodies,  and 
frequently  spoken  of  as  the  purin  bases. 

Indol,  Skatol,  Phenol,  Cresol. — These  compounds,  products  of  the  putre- 
factive changes  in  the  derivatives  of  protein  are  present  in  variable  amounts, 
associated  with  potassium  sulphate  (see  page  i6o).  These  compounds  are 
known  as  the  ethereal  sulphates.  The  extent  to  which  they  are  present  is 
taken  as  a  measure  of  the  extent  of  intestinal  putrefaction. 


144  HUMAN  PHYSIOLOGY 

Inorganic  Salts. — Sodium  and  potassium  phosphates,  known  as  the 
alkaline  phosphates,  are  found  in  both  blood  and  urine.  The  total  quan- 
tity excreted  daily  is  about  4  grams.  Calcium  and  magnesium  phosphates, 
known  as  the  eany  phosphates j  are  present  to  the  extent  of  i  gram.  Though 
insoluble  in  water,  they  are  held  in  solution  in  the  urine  by  its  acid  con- 
stituents. If  the  urine  be  rendered  alkaline,  they  are  at  once  precipitated. 
Sodium  and  potassium  sulphate  are  also  present  to  the  extent  of  about  2 
grams.  The  phosphoric  and  sulphuric  acids  which  are  combined  with 
these  bases  enter  the  body  for  the  most  part  in  the  foods,  though  there  is 
evidence  that  they  also  arise  by  oxidation  in  consequence  of  the  metabol- 
ism of  proteins  which  contain  phosphorus  and  sulphur.  Sodium  chlorid  is 
the  most  abundant  of  the  inorganic  salts.  It  is  derived  mainly  from  the 
food.     The  amount  excreted  is  about  15  grams  in  twenty-four  hours. 

KIDNEYS 

The  kidneys  are  the  organs  for  the  secretion  of  urine.  They  are  situated 
in  the  lumbar  region,  one  on  each  side  of  the  vertebral  column  behind  the 
peritoneum,  and  extend  from  the  eleventh  rib  to  the  crest  of  the  ilium;  the 
anterior  surface  is  convex,  the  posterior  surface  concave,  the  latter  present- 
ing a  deep  notch,  the  hilus.  » 

The  kidney  is  surrounded  by  thin,  smooth  membrane  composed  of  white 
fibrous  and  yellow  elastic  tissue;  though  it  is  attached  to  the  surface  of  the 
kidney  by  minute  processes  of  connective  tissue,  it  can  be  readily  torn 
away.    The  substance  of  the  kidney  is  dense,  but  friable. 

Upon  making  a  longitudinal  section  of  the  kidney  it  will  be  observed  that 
the  hilus  extends  into  the  interior  of  the  organ  and  expands  to  form  a  cavity 
known  as  the  sinus.  This  cavity  is  occupied  by  the  upper,  dilated  portion 
of  the  ureter,  the  interior  of  which  forms  the  pelvis.  The  ureter  subdivides 
into  several  portions,  which  ultimately  give  origin  to  a  number  of  smaller 
tubes,  termed  calcyes,  which  receive  the  apices  of  the  pyramids  (Fig.  13). 

The  parenchyma  of  the  kidney  consists  of  two  portions — viz. : 

1,  An  internal  or  medullary  portion,  consisting  of  a  series  of  pyramids  or 
cones,  some  twelve  or  fifteen  in  number.  They  present  a  distinctly  striated 
appearance,  a  condition  due  to  the  straight  direction  of  the  tubules  and 
blood  vessels. 

2.  An  external  or  cortical  portion,  consisting  of  a  delicate  matrix  contain- 
ing an  immense  number  of  tubules  having  a  markedly  convoluted  appear- 
ance. Throughout  its  structure  are  found  numerous  small  ovoid  bodies, 
termed  Malpighian  corpuscles. 


KIDNEYS 


145 


The  Uriniferous  Tubules. — The  kidney  is  a  compound,  tubular  gland 
composed  of  microscopic  tubules  whose  function  it  is  to  secrete  from  the 
blood  those  waste  products  which  collectively  constitute  the  urine.     If  the 


Fig.  13. — Longitudinal  Section  through  the  Kidney,  the  Pelvis  of  the 
Kidney,  and  a  Number  of  Renal  Calyces. — {Tyson,  after  Henle.) 
A.  Branch  of  the  renal  artery.  U.  Ureter.  C.  Renal  calyx,  i.  Cortex.  1'. 
Medullary  rays.  1".  Labyrinth,  or  cortex  proper.  2.  Medulla.  2'.  Papillary 
portion  of  medulla,  or  medulla  proper.  2".  Border  layer  of  the  medulla.  3.  3* 
Transverse  section  through  the  axes  of  the  tubules  of  the  border  layer.  4.  Fat  of 
the  renal  sinus,     s,  5.  Arterial  branches.       *  Transversely  coursing  medulla   rays. 


apex  of  each  pyramid  be  examined  with  a  lens,  it  will  present  a  number  of 
small  orifices,  which  are  the  beginning  of  the  uriniferous  tubules.  From 
this  point  the  tubules  pass  outward  in  a  straight  but  somewhat  divergent 
manner  toward  the  cortex,  giving  off  at  acute  angles  a  number  of  branches 
10 


146 


HUMAN  PHYSIOLOGY 


(Fig.  14).  From  the  apex  to  the  base  of  the  pyramids  they  are  known  as  the 
tubules  of  Bellini.  In  the  cortic^  portion  of  the  kidney  each  tubule  be- 
comes enlarged  and  twisted,  and  after  pursuing  an  extremely  convoluted 
course,  turns  backward  into  the  medullary  portion  for  some  distance, 
forming  the  descending  limb  of  Henle's  loop:  it  then  turns  upon  itself, 
forming  the  ascending  limb  of  the  loop,  reenters  the  cortex,  again  expands, 
and  finally  terminates  in  a  spheric  enlargement  known  as  Mailer's  or 
Bowman's  capsule.  Within  this  capsule  is  contained  a  small  tuft  of  blood- 
vessels, constituting  the  glomerulus,  or  Malpighian 
corpuscles. 

Structure  of  the  Tubules. — Each  tubule  consists 
of  a  basement  membrane  lined  by  epithelium  cells 
throughout  its  entire  extent.  The  tubule  and  its 
contained  epithelium  vary  in  shape  and  size  in  dif- 
ferent parts  of  its  course.  The  termination  of  the 
convoluted  tube  consists  of  a  little  sac  or  capsule, 
which  is  ovoid  in  shape  and  measures  about  J^oo 
of  an  inch.  This  capsule  is  lined  by  a  layer  of 
flattened  epithelial  cells,  which  is  also  reflected 
over  the  surface  of  the  glomerulus.  During  the 
periods  of  secretory  activity  the  blood-vessels  of 
the  glomerulus  become  filled  with  blood,  so  that 
the  cavity  of  the  sac  is  almost  obliterated;  after 
secretory  activity  the  blood-vessels  contract  and 
the  sac-cavity  becomes  enlarged.  In  that  portion 
of  the  tubule  lying  between  the  capsule  and  Henle's 
loop  the  epithelial  cells  are  cuboid  in  shape;  in 
Henle's  loop  they  are  flattened,  while  in  the  re- 
mainder of  the  tubule  they  are  cuboid  and 
columnar. 


Fig.  14. — Diagram- 
matic Exposition  of 
THE  Method  in  which 

THE  UrINIFEROUS  TUBES 

Unite  to  Form  Primi- 
tive Cones. — {Tyson, 
after  Ludwig.) 


Blood-vessels  of  the  Kidney. — The  renal  artery 
is  of  large  size  and  enters  the  organ  at  the  hilum; 
it  divides  into  several  large  branches,  which  penetrate  the  substance 
of  the  kidney  between  the  pyramids,  at  the  base  of  which  they  form 
an  anastomosing  plexus,  which  completely  surrounds  them.  From 
this  plexus  vessels  follow  the  straight  tubes  toward  the  apex  of  the 
pyramids,  while  others  enter  the  cortical  portion  and  pass  to  the 
surface.  In  the  course  of  the  latter,  small  branches  are  given  off,  each  of 
which  soon  divides  and  subdivides  to  form  a  ball  of  capillary  vessels 
known  as  the  glomerulus.     These  capillaries,  however,  do  not  anastomose, 


KIDNEYS  147 

but  soon  reunite  to  form  an  efferent  vessel  the  caliber  of  which  is  less  than 
that  of  the  afferent  artery.  In  consequence  of  this,  there  is  a  greater  re- 
sistance to  the  outflow  of  blood  than  to  the  inflow,  and,  therefore,  a  higher 
blood-pressure  in  the  glomerulus  than  in  capillaries  generally.  The  rela- 
tion of  the  glomerulus  to  the  tubule  is  important  from  a  physiologic  point 
of  view.  As  stated  above,  the  glomerulus  is  received  into  and  surrounded 
by  the  terminal  expansion  or  capsule  of  the  tubule.  This  capsule,  formed 
by  an  invagination  of  the  terminal  portion  of  the  tubule,  consists  of  two 
walls,  an  outer  one  consisting  of  an  extremely  thin  basement  membrane, 
covered  by  flattened  epithelial  cells,  and  an  inner  one  consisting  appar- 
ently only  of  flattened  epithelium  which  is  reflected  over  and  closely 
invests  the  glomerular  blood-vessels.  The  blood  is  thus  separated  from  the 
interior  of  the  capsule  by  the  epithelial  wall  of  the  capillary  and  the  epithe- 
lium of  the  reflected  wall  of  the  capsule.  After  its  exit  from  the  capsule 
the  efferent  vessel  of  the  glomerulus  soon  again  divides  and  subdivides 
to  form  an  elaborate  capillary  plexus  which  surrounds  and  closely  invests 
the  convoluted  tubules.  From  this  plexus  as  well  as  from  the  plexus  which 
surrounds  the  straight  tubules  veins  arise  which  pass  toward  and  empty 
into  veins  at  the  base  of  the  pyramids.  The  renal  vein  formed  by  the 
union  of  these  latter  veins  emerges  from  the  kidney  at  the  hilum  and  finally 
empties  into  the  vena  cava  inferior. 

The  nerves  to  the  kidney  have  their  origin  in  the  cells  of  small  ganglia 
situated  close  to  the  semilunar  ganglion.  They  pass  to  the  kidney  in  the 
renal  plexus  and  follow  the  course  of  the  blood-vessels  to  their  termina- 
tion. The  small  renal  ganglia  are  in  connection  with  the  spinal  cord  by 
means  of  the  small  splanchnics.  The  nerve  fibers  have  both  vaso-con- 
strictor  and  vaso-dilatator  functions. 

The  Renal  Duct. — The  renal  duct,  the  ureter,  is  a  membranous  tube,  sit- 
uated behind  the  peritoneum  about  the  diameter  of  a  goose-quill,  eighteen 
inches  in  length,  and  extends  from  the  pelvis  of  the  kidney  to  the  base  of 
the  bladder,  which  it  perforates  in  an  oblique  direction.  It  is  composed 
of  three  coats:  fibrous,  muscle  and  mucous. 

Mechanism  of  Urine  Formation. — Inasmuch  as  the  kidney  presents  (i) 
an  apparatus  for  filtration,  the  capsule  with  its  enclosed  glomerulus,  and 
(2)  an  apparatus  for  secretion,  the  tubule  with  its  epithelium,  it  was 
originally  inferred  by  Bowman  that  the  elimination  of  the  constituents 
of  the  urine  from  the  blood  is  accomplished  by  the  two-fold  process  of 
filtration  and  secretion;  that  the  water  and  highly  diffusible  inorganic  salts 
simply  pass  by  diffusion  through  the  walls  of  the  blood-vessels  of  the 
glomerulus  into  the  capsule  of  Miiller,  while  the  urea  and  remaining  organic 


148  HUMAN  PHYSIOLOGY 

constituents  are  removed  by  true  secretory  action  of  the  renal  epithelium. 
Modern  experimentation  supports  this  view  of  renal  action  though  subject 
to  some  modification. 

The  progress  of  physiologic  investigation  has  confirmed  the  view  that  the 
capsule  and  glomerulus  form  a  passive  apparatus  for  the  passage  of  a  fil- 
trate not  merely  of  water  and  inorganic  salts,  but  having  the  characteris- 
tics and  composition  of  the  blood  plasma  less  its  protein  content.  That 
the  epithelium  is  not  only  a  secretory  apparatus  removing  organic  constit- 
uents from  the  blood  but  is  also  an  absorptive  apparatus  whereby  water 
and  inorganic  salts  may  be  returned  to  the  blood  when  needed  for  nutritive 
purposes.  The  physical  properties  and  chemic  composition  of, urine  are 
resultants  of  the  cooperative  action  of  these  different  factors. 

The  Influence  of  Blood  Pressure. — The  filtration  of  urinary  constit- 
uents from  the  glomerulus  into  Miiller's  capsule  depends  largely  upon  the 
blood-pressure  and  the  rapidity  of  blood  flow  in  the  renal  artery  and 
glomerulus. 

The  pressure  of  the  blood  in  the  glomeruli  may  be  raised  and  the  velocity 
increased : 

1.  By  an  increase  in  blood-pressure  generally. 

2.  By  an  increase  in  the  pressure  of  the  renal  artery  alone. 

The  first  condition  may  be  brought  about  by  an  increase  in  either  the 
force  or  frequency  of  the  heart's  action  or  by  a  contraction  of  the  arterioles 
of  vascular  areas  in  any  or  all  parts  of  the  body,  excepting,  of  course,  the 
renal  vascular  area.  The  second  condition  is  brought  about  by  a  dilata- 
tion of  the  renal  artery  alone  and  possibly  by  a  contraction  of  the  efferent 
vessels  of  the  glomeruli. 

The  pressure  of  the  blood  in  the  glomeruli  may  be  diminished  and  the 
velocity  decreased : 

1.  By  a  decrease  in  the  blood-pressure  generally. 

2.  By  a  decrease  in  the  pressure'of  the  renal  artery  alone. 

The  first  condition  is  brought  about  by  a  decrease  in  either  the  force  or 
frequency  of  the  heart's  action  or  by  a  dilatation  of  the  arterioles  of  large 
vascular  areas  in  any  or  all  parts  of  the  body.  The  second  condition  is 
brought  about  by  contraction  of  the  renal  artery  alone  and  possibly  by  a 
dilatation  of  the  efferent  vessels  of  the  glomeruli.  Coincident  with  the 
rise  and  fall  of  pressure  in  the  glomerular  capillaries  there  is  a  rise  and  fall 
in  the  rate  of  urinary  flow. 

The  Storage  and  Discharge  of  Urine.— Urination.— The  urinary  con- 
stituents, as  soon  as  they  are  eliminated  from  the  blood,  pass  into  and 
through  the  uriniferous  tubules  and  by  them  are  discharged  into  the  pelvis 


SKIN  149 

of  the  kidney.  They  then  enter  the  ureter  by  which  they  are  conducted  to 
the  bladder.  The  immediate  cause  of  this  movement  is  undoubtedly  a 
difference  of  pressure  between  the  terminal  portions  of  the  tubules  and  the 
terminal  portion  of  the  ureter,  aided  by  the  peristaltic  contraction  of  the 
muscle  wall  of  the  ureter. 

The  Bladder. — The  bladder  is  a  reservoir  for  the  reception  and  tempo 
rary  storage  of  the  urine  prior  to  its  expulsion  from  the  body;  when  fully 
distended  it  is  ovoid  in  shape,  and  holds  from  600  to  800  c.c.  It  is  com- 
posed of  four  coats;  serous,  muscle  (the  fibers  of  which  are  arranged 
longitudinally  and  circularly),  areolar,  and  mucous.  The  orifice  of  the 
bladder  is  controlled  by  the  sphincter  vesica,  a  muscular  band  about  ^2  of 
an  inch  in  width.  The  muscle-fibers  collectively  constitute  the  detrusor 
urinae  muscle. 

Nerve  Mechanism  of  Urination. — When  the  urine  has  passed  into  the 
bladder,  it  is  there  retained  by  the  sphincter  vesicae  muscle,  kept  in  a 
state  of  tonic  contraction  by  the  action  of  a  nerve  center  in  the  lumbar 
region  of  the  spinal  cord.  This  center  can  be  inhibited  and  the  sphincter 
relaxed,  either  rejlexly,  by  impressions  coming  through  sensory  nerves 
from  the  mucous  membrane  of  the  bladder,  or  directly,  by  a  voluntary 
impulse  descending  the  spinal  cord.  When  the  desire  to  urinate  is 
experienced,  impressions  made  upon  the  vesical  sensory  nerves  are  carried 
to  the  centers  governing  the  sphincter  and  detrusor  urince  muscles  and  to 
the  brain.  If  now  the  act  of  urination  is  to  take  place,  a  voluntary 
impulse  originating  in  the  brain  passes  down  the  spinal  cord  and  still 
further  inhibits  the  sphincter  vesicae  center,  with  the  effect  of  relaxing  the 
muscle  and  of  stimulating  the  center  governing  the  detrusor  muscle, 
with  the  effect  of  contracting  the  muscle  and  expelling  the  urine.  If 
the  act  is  to  be  suppressed,  voluntary  impulses  inhibit  the  detrusor  center 
and  possibly  stimulate  the  sphincter  center. 

The  genitospinal  center  controlling  these  movements  is  situated  in  that 
portion  of  the  spinal  cord  corresponding  to  the  origin  of  the  third,  fourth, 
and  fifth  sacral  nerves. 

SKIN 

The  Skin. — The  skin,  the  external  investment  of  the  body,  is  a  most 
complex  and  important  structure,  serving — 

PI.  As  a  protective  covering. 
2.  As  an  organ  for  tactile  sensibility. 

3.  As  an  organ  for  the  elimination  of  excrementitious  matters. 
The  amount  of  skin  investing  the  body  of  a  man  of  average  size  is  about 


150  HUMAN  PHYSIOLOGY 

twenty  feet,  and  varies  in  thickness,  in  different  situations,  from  }i  to  Hoo 
of  an  inch. 

The  skin  consists  of  two  principal  layers — viz.,  a  deeper  portion,  the 
corium,  and  a  superficial  portion,  the  epidermis. 

The  Coriiun. — The  corium,  or  cutis  vera,  may  be  subdivided  into  a 
reticulated  and  a  papillary  layer.  The  former  is  composed  of  white  fibrous 
tissue,  non-striated  muscle-fibers,  and  elastic  tissue,  interwoven  in  every 
direction,  forming  an  areolar  network,  in  the  meshes  of  which  are  deposited 
masses  of  fat,  and  a  structureless,  amorphous  matter;  the  latter  is  formed 
mainly  of  club-shaped  elevations  or  projections  of  the  amorphous  matter, 
constituting  the  papillce;  they  are  most  abundant  and  well  developed 
under  the  palms  of  the  hands  and  upon  the  soles  of  the  feet;  they  average 
Jfoo  of  an  inch  in  length,  and  may  be  simple  or  compound;  they  are  well 
supplied  with  nerves,  blood  vessels,  and  lymphatics. 

The  Epidermis. — The  epidermis,  or  scarf  skin,  is  an  extravascular 
structure,  a  product  of  the  true  skin,  and  is  composed  of  several  layers  of 
cells.  It  may  be  divided  into  two  layers:  the  rete  mucosum,  or  the 
Malpighian  layer,  and  the  horny  or  corneous. 

The  former  is  closely  adherent  to  the  papillary  layer  of  the  true  skin,  and 
is  composed  of  large  nucleated  cells,  the  lowest  layer  of  which,  the  **  prickle 
cells,'*  contains  pigment- granules,  which  give  to  the  skin  its  varying  tints 
in  different  individuals  and  in  different  races  of  men;  the  more  superficial 
cells  are  large,  colorless,  and  semi-transparent.  The  latter,  the  corneous 
layer,  is  composed  of  flattened  cells,  which,  from  their  exposure  to  the 
atmosphere,  are  hard  and  horny  in  texture;  it  varies  in  thickness  from  J^  of 
an  inch  on  the  palms  of  the  hands  and  soles  of  the  feet  to  J^oo  of  an  inch 
in  the  external  auditory  canal. 

Appendages  of  the  Skin. — ^Hairs  are  found  in  almost  all  portions  of  the 
body,  and  can  be  divided  into — 

1.  Long,  soft  hairs,  on  the  head. 

2.  Short,  stiff  hairs,  along  the  edges  of  the  eyelids  and  nostrils. 

3.  Soft,  downy  hairs  on  the  general  cutaneous  surface. 

They  consist  of  a  root  and  a  shaft.  The  latter  is  oval  in  shape  and  about 
J^oo  of  an  inch  in  diameter;  it  consists  of  fibrous  tissue,  covered  externally 
by  a  layer  of  imbricated  cells,  and  internally  by  cells  containing  granular 
and  pigment  material. 

The  root  of  the  hair  is  embedded  in  the  hair-follicle,  formed  by  a  tubular 
depression  of  the  skin,  extending  nearly  through  to  the  subcutaneous 
tissue;  its  walls  are  formed  by  the  layers  of  the  corium,  covered  by  epi- 
dermic cells.    At  the  bottom  of  the  follicle  is  a  papillary  projection  of 


SKIN  151 

amorphous  matter,  corresponding  to  a  papilla  of  the  true  skin,  containing 
blood-vessels  and  nerves,  upon  which  the  hair-root  rests.  The  invest- 
ments of  the  hair-roots  are  formed  of  epithelial  cells,  constituting  the 
internal  and  external  root-sheaths. 

The  hair  protects  the  head  from  the  heat  of  the  sun  and  from  the  cold, 
retains  the  heat  of  the  body,  prevents  the  entrance  of  foreign  matter  into 
the  lungs,  nose,  ears,  etc.  The  color  is  due  to  pigment  matter.  In  old 
age  the  hair  becomes  more  or  less  whitened. 

The  Sebaceous  Glands. — The  sebaceous  glands,  embedded  in  the 
true  skin,  are  simple  and  compound  racemose  glands,  opening,  by  a 
common  excretory  duct,  upon  the  surface  of  the  epidermis  or  into  the 
hair-follicle.  They  are  found  in  all  portions  of  the  body,  most  abundantly 
in  the  face,  and  are  formed  by  a  delicate,  structureless  membrane,  lined 
by  flattened  polyhedral  cells-  The  sebaceous  glands  secrete  a  peculiar 
oily  matter  (the  sebum),  by  which  the  skin  is  lubricated  and  the  hairs 
are  softened;  it  is  quite  abundant  in  the  region  of  the  nose  and  forehead, 
which  often  presents  a  greasy,  glistening  appearance;  it  consists  of  water, 
mineral  salts,  fatty  globules,  and  epithelial  cells. 

The  vernix  caseosa,  which  frequently  covers  the  surface  of  the  fetus  at 
birth,  consists  of  the  residue  of  the  sebaceous  matter,  containing  epithelial 
cells  and  fatty  matters;  it  seems  to  keep  the  skin  soft  and  supple,  and 
guards  it  from  the  ejffects  of  the  long-continued  action  of  the  amniotic 
water. 

The  Sudoriparous  Glands. — The  sudoriparous  glands  excrete  the 
sweat.  They  consist  of  a  mass  or  coil  of  a  tubular  gland  duct,  situated 
in  the  derma  and  in  the  subcutaneous  tissue,  average  3^5  of  an  inch  in 
diameter,  and  are  surrounded  by  a  rich  plexus  of  capillary  blood-vessels. 
From  this  oil  the  duct  passes  in  a  straight  direction  up  through  the  skin 
to  the  epidermis,  where  it  makes  a  few  spiral  turns  and  opens  obliquely 
upon  the  surface.  The  sweat-glands  consist  of  a  delicate  homogeneous 
membrane  lined  by  epithelial  cells,  whose  function  is  to  extract  from  the 
blood  the  elements  existing  in  the  perspiration. 

The  glands  are  very  abundant  all  over  the  cutaneous  surface — as  many 
as  3,528  to  the  square  inch,  according  to  Erasmus  Wilson. 

The  perspiration  is  an  excrementitious  fluid,  clear,  colorless,  almost 
odorless,  slightly  acid  in  reaction,  with  a  specific  gravity  of  1,003  to  1,004. 

The  total  quantity  of  perspiration  excreted  daily  has  been  estimated 
at  about  two  pounds,  though  the  amount  varies  with  the  nature  of  the 
food  and  drink,  exercise,  external  temperature,  season,  etc. 

The  elimination  of  the  sweat  is  not  intermittent,  but  continuous:  it 


152  HUMAN  PHYSIOLOGY 

takes  place  so  gradually  that  as  fast  as  it  is  formed  it  passes  off  by  evapo- 
ration as  insensible  perspiration.  Under  exposure  to  great  heat  and 
exercise  the  evaporation  is  not  sufficiently  rapid,  and  it  appears  as  sensible 
perspiration. 

Composition  of  Sweat 

Water 995  573 

Urea o .  043 

Fatty  matters    0.014 

Alkaline  lactates *♦,.•••  0.317 

Alkaline  sudorates i .  562 

Inorganic  salts 2 .  491 

1,000.000 

Urea  is  a  constant  ingredient. 

Carbonic  acid  is  also  exhaled  from  the  skin,  the  amount  being  about 
3^0  0  of  that  from  the  lungs. 

Prespiration  regulates  the  temperature  and  removes  waste  matters 
from  the  blood;  it  is  so  important  that  if  elimination  be  prevented,  death 
occurs  in  a  short  time. 

Influence  of  the  Nerve  System. — The  secretion  of  sweat  is  regulated 
by  the  nerve  system.  Here,  as  in  the  secreting  glands,  the  fluid  is  formed 
from  material  in  the  lymph-spaces  surrounding  the  gland.  Two  sets  of 
nerves  are  concerned — viz.:  vasomotor,  regulating  the  blood-supply;  and 
secretor,  stimulating  the  activities  of  the  gland  cells.  Generally  the  two 
conditions,  increased  blood  flow  and  increased  glandular  action,  coexist. 
At  times  profuse  clammy  perspiration  occurs,  with  diminished  blood  flow. 
Sweat  centers  are  found  in  the  spinal  cord  between  the  levels  of  the 
second  thoracic  and  third  lumbar  nerves.  The  secretory  fibers  reach 
the  perspiratory  glands  of  the  head  and  face  through  the  cervical  sympa- 
thetic; of  the  arms,  through  the  thoracic  sympathetic,  ulnar,  and  radial 
nerves;  of  the  leg,  through  the  abdominal  sympathetic  and  sciatic  nerves. 
The  course  they  pursue  is  similar  to  those  of  the  vasomotor  nerves  with 
which  they  are  associated. 

The  sweat-center  is  excited  to  action  by  mental  emotions,  increased 
temperature  of  blood  circulating  in  the  medulla  and  cord,  increased 
venosity  of  blood,  many  drugs,  rise  of  external  temperature,  exercise^  etc. 

EXTERNAL  SECRETIONS 

Secretion  is  a  term  applied  to  a  process  by  which  complex  fluids  are 
formed  from  the  constituents  of  the  lymph  which  are  separated  from  the 


EXTERNAL   SECRETIONS  153 

blood-stream  by  the  activities  of  the  endothelial  cells  of  the  capillary  wall, 
as  the  blood  flows  through  the  capillary  blood-vessels. 

These  separated  materials  may  be  utilized  in  several  ways: 

1.  For  the  repair  of  the  tissues,  for  growth,  for  the  liberation  of  energy. 

2.  For  the  elaboration  or  production  by  specialized  organs  of  a  variety 
of  complex  fluids  and  specific  materials,  of  widely  different  application. 
The  fluids  and  specific  materials  thus  formed  are  utilized  for  the  most 
part  to  meet  some  special  need  of  the  body.  All  such  fluids  and  mate- 
rials are  termed  secretions^  and  the  organs  by  which  they  are  formed  are 
termed  secretor  organs.  Secretions  whether  simple  or  complex  may  in 
a  general  way  be  divided  into  two  groups,  viz.:  external  and  internal. 

External  Secretions. — An  external  secretion  may  be  defined  as  a  more 
or  less  complex  fluid  formed  by  the  secretor  activities  of  epithelial  cells  of 
glands,  which  is  discharged  through  well-defined  ducts  on  the  surfaces  of 
the  body,  the  skin  or  mucous  membrane.  The  glands  by  which  they  are 
formed  or  secreted  are  known  as  glands  of  external  secretion. 

Internal  Secretions. — Internal  secretions  may  be  defined  as  more  or 
less  complex  materials  or  agents  formed  by  the  activities  of  epithelial 
cells  of  organs,  and  which  are  discharged  into,  and  distributed  by  the  blood 
to  organs  and  tissues  near  and  remote,  the  activities  of  which  they  influence 
in  varying  ways  and  degrees.  The  glands  by  which  they  are  formed  or 
secreted  are  known  as  glands  of  internal  secretion. 

Organs  of  External  Secretions. — All  organs  belonging  to  this  group 
consist  primarily  of  a  thin  delicate  homogeneous  membrane,  one  side  of 
which  is  covered  with  a  layer  of  epithelial  cells  and  the  other  side  of  which 
is  closely  invested  by  a  network  of  capillary  blood-vessels ,  lymph-vessels, 
and  nerves.  Though  the  epithelial  cells  have  a  general  histologic  resem- 
blance one  to  another,  their  physiologic  function  varies  in  different  situa- 
tions, in  accordance  probably  with  their  ultimate  chemic  structure,  a  fact 
which  determines  the  difference  in  the  character  of  the  secretions. 

These  organs  may  consist  of  a  single  layer  of  cells  or  a  group  of  cells,  and 
may  be  subdivided  into — 

1.  Secreting  membranes. 

2.  Secreting  glands. 

The  secreting  membranes  are  the  mucous  membranes  lining  the 
gastro-intestinal,  the  pulmonary,  and  the  geni to-urinary  tracts.  The 
secreting  glands  are  formed  of  the  same  histologic  elements  as  the  secreting 
membranes.  They  are  formed  by  an  involution  of  the  mucous  membrane 
or  skin,  the  epithelium  of  which  is  variously  modified  structurally  and 


154  HUMAN  PHYSIOLOGY 

functionally  in  the  various  situations  in  which  they  are  formed.  Like 
the  membranes  themselves,  the  glands  are  invested  by  capillary  blood- 
vessels and  supplied  with  lymph-vessels  and  nerves,  of  which  the  latter 
are  in  direct  connection  with  the  blood-vessels  and  epithelial  cells.  The 
interior  of  each  gland  is  in  communication  with  the  free  surface  by  one 
or  more  passageways  known  as  ducts. 

These  glands  may  be  classified  according  as  the  involution  is  cylindrical 
or  dilated  as — 

1.  Tubular.  The  tubular  glands  may  be  simple — e.g.,  sweat-glands, 
intestinal  glands,  fundus  glands  of  the  stomach;  or  compound — e.g., 
kidney,  testicle,  salivary,  and  lachrymal  glands. 

2.  Alveolar.  •  The  alveolar  glands  may  also  be  simple — e.g.y  the  seba- 
ceous glands,  the  ovarian  follicles,  meibomian  glands;  or  compound,  as 
the  mammary  glands  and  salivary  glands. 

In  the  production  of  the  secretion  two  essentially  different  processes 
are  concerned: 

1.  Chemic. — The  formation  and  elaboration  of  the  characteristic  organic 
ingredients  of  the  secreted  fluids — e.g.y  pepsin,  pancreatin — take  place 
during  the  intervals  of  glandular  activity,  as  a  part  of  the  general  function 
of  nutrition.  They  are  formed  by  the  cells  lining  the  glands,  and  can 
often  be  seen  in  their  interior  with  the  aid  of  the  microscope — e.g.,  bile  in 
the  liver-cells,  fat  in  the  cells  of  the  mammary  gland. 

2.  Physical. — Consisting  of  a  transudation  of  water  and  mineral  salts 
from  the  blood  into  the  interior  of  the  gland. 

During  the  intervals  of  glandular  activity  only  that  amount  of  blood 
passes  through  the  gland  sufficient  for  proper  nutrition;  when  the  gland 
begins  to  secrete,  under  the  influence  of  an  appropriate  stimulus,  the  blood- 
vessels dilate  and  the  quantity  of  blood  becomes  increased  beyond  that 
flowing  to  the  gland  during  its  repose. 

Under  these  conditions  a  transudation  of  water  and  salt  takes  place, 
washing  out  the  characteristic  ingredients,  which  are  discharged  by  the 
gland  ducts.  The  discharge  of  the  secretion  is  intermittent;  they  are 
retained  in  the  glands  until  they  receive  the  appropriate  stimulus,  when 
they  pass  into  the  larger  ducts  by  the  vis  a  tergo,  and  are  then  discharged 
by  the  contraction  of  the  muscular  walls  of  the  ducts. 

The  activity  of  glandular  secretion  is  hastened  by  an  increase  in  the 
blood-volume  and  pressure  and  retarded  by  a  diminution. 

The  Influence  of  the  Nerve  System. — The  activity  of  every  gland  is 
controlled  by  nerve-centers  situated  in  the  central  nerve-system.     These 


MAMMARY  GLANDS  1 55 

centers  may  be  excited  to  activity  either  by  impressions  made  on  the 
peripheral  terminations  of  afferent  nerves  or  by  emotional  states;  or, 
possibly,  by  changes  in  the  composition  of  the  blood  itself.  As  a  rule, 
all  normal  secretion  is  a  reflex  act  involving  the  usual  mechanism,  viz.: 
a  receptive  surface  (skin,  mucous  membrane,  or  sense-organ),  an  afferent 
nerve,  an  emissive  cell  from  which  emerges  an  efferent  nerve  to  be  dis- 
tributed to  a  responsive  organ,  the  gland  epithelium,  though  the  secretion 
may  in  some  instances  be  initiated  by  a  psychic  state. 

The  structure  of  the  glands  of  external  secretion,  the  composition  and 
physiologic  actions  of  their  secretions  have  in  large  part  been  considered 
in  the  foregoing  chapter  on  Digestion.  There  remains,  however,  to  be 
considered  the  mammary  glands,  the  liver  and  the  sebaceous  glands. 


MAMMARY  GLANDS 

The  mammary  glands,  which  secrete  the  milk,  are  two  more  or  less 
hemispheric  organs,  situated  in  the  human  female  on  the  anterior  surface 
of  the  thorax.  Though  rudimentary  in  childhood,  they  gradually  increase 
in  size  as  the  young  female  approaches  puberty. 

The  gland  presents  at  its  convexity  a  small  prominence  of  skin  (the  nipple) 
which  is  surrounded  by  a  circular  area  of  pigmented  skin  (the  areola). 
The  gland  proper  is  covered  by  a  layer  of  adipose  tissue  anteriorly  and  is 
attached  posteriorly  to  the  pectoral  muscles  by  a  meshwork  of  fibrous 
tissue.  During  utero-gestation  the  mammary  glands  become  larger,  firmer, 
and  more  lobulated;  the  areola  darkens  and  the  veins  become  more  promi- 
nent. At  the  period  of  lactation  the  gland  is  the  seat  of  active  histologic 
and  physiologic  changes,  correlated  with  the  production  of  milk.  At 
the  close  of  lactation  the  glands  diminish  in  size,  undergo  involution,  and 
gradually  return  to  their  original  non-secreting  condition. 

Structure  of  the  Mammary  Gland. — The  mammary  gland  consists  of 
an  aggregation  of  some  fifteen  or  twenty  lobes,  each  of  which  is  surrounded 
by  a  framework  of  fibrous  tissue.  The  lobe  is  provided  with  an  excretory 
duct,  which,  as  it  approaches  the  base  of  the  nipple,  expands  to  form  a 
sinus  or  reservoir,  beyond  which  it  opens  by  a  narrowed  orifice  on  the 
surface  of  the  nipple.  On  tracing  the  duct  into  a  lobe,  it  is  found  to  divide 
and  subdivide,  and  finally  to  terminate  in  lobules  or  acini.  Each  acinus 
consists  of  a  basement  membrane,  lined  by  low  polyhedral  cells.  Exter- 
nally it  is  surrounded  by  connective  tissue  supporting  blood-vessels, 
lymphatics  and  nerves. 


156 


HUMAN  PHYSIOLOGY 


MILK 

Milk  is  an  opaque,  bluish-white  fluid,  almost  inodorous,  of  a  sweet 
taste,  an  alkaline  reaction,  and  a  specific  gravity  of  1,025  to  1,040.  When 
examined  microscopically  it  is  seen  to  consist  of  a  clear  fluid  (the  milk- 
plasma),  holding  in  suspension  an  enormous  number  of  small,  highly 
refractive  oil-globules,  which  measure,  on  an  average,  Ko 0,0 00  of  an  inch 
in  diameter.  Each  globule  is  supposed  by  some  observers  to  be  surrounded 
by  a  thin,  albuminous  envelope,  which  enables  it  to  maintain  the  discrete 
form.  The  quantity  of  milk  secreted  daily  by  the  human  female  averages 
about  two  and  a  half  pints.  The  milk  of  all  mammalia  consists  of  all 
the  different  classes  of  nutritive  principles,  though  in  varying  proportions.. 
The  relative  proportions  in  which  these  constituents  exist  are  shown  in 
the  following  table  of  analyses: 

The  Composition  of  Milk 


Constituents 

Human 

Cow 

Goat 

Mare 

Ass 

Water 

87.80 

I. SO 

3. SO 
7.00 
0.20 

87.00 

3.20 

3.80 
5. 00 
O.SO 

86.91 

3.69 

409 
4.4s 
0.86 

90.00 
1.80 
1.30 

s.so 

0.30 

90.00 

Caseinogen 

2 .  10 

Lactalbumin      / 

Fat 

1 .30 

Lactose 

6.30 

Inorganic  Salts 

0.30 

Caseinogen  is  the  chief  protein  constituent  of  milk,  and  is  held  in  solution 
by  the  presence  of  calcium  phosphate.  On  the  addition  of  acetic  acid  or 
of  sodium  chlorid  up  to  the  point  of  saturation,  the  caseinogen  is  precipi- 
tated as  such,  and  may  be  collected  by  appropriate  chemic  methods.  When 
taken  into  the  stomach  caseinogen  is  coagulated — that  is,  it  is  separated 
into  casein  or  tyrein  and  a  small  quantity  of  a  new  soluble  protein.  The 
ferment  which  induces  this  change  is  known  as  rennin.  The  presence  of 
calcium  phosphate  is  necessary  for  this  coagulation. 

Fat  is  present  in  the  condition  of  a  fine  emulsion  and  is  more  or  less 
solid  at  ordinary  temperatures.  It  is  a  composition  of  olein,  palmitin, 
and  stearin,  with  a  small  quantity  of  butyrin  and  caproin.  When  milk 
is  allowed  to  stand  for  some  time  the  fat-globules  rise  to  the  surface  and 
form  a  thick  layer,  known  as  cream.  When  subjected  to  the  churning 
process,  the  fat  globules  run  together  and  form  a  cohesive  mass — the 
butter. 


MILK  157 

Lactose  is  the  particular  form  of  sugar  characteristic  of  milk.  It  be- 
longs to  the  saccharose  group  and  has  the  following  composition:  Ci2H2j- 
Oii.  In  the  presence  of  the  Bacilus  acidi  lacticl  the  lactose  is  in  part 
reduced  to  lactic  acid  and  carbon  dioxid,  the  former  of  which  will  cause  a 
precipitation  of  the  caseinogen.  It  is  the  presence  of  lactic  acid  that 
imparts  the  sour  taste  to  milk. 

Inorganic  salts  are  always  present  and  are  chiefly  those  of  potassium, 
sodium,  calcium,  and  magnesium,  phosphates  and  chlorids. 

Iron  is  also  present  in  small  amounts  possibly  from  3  to  5  milligrams  per 
1,000  c.c.     Citric  acid  to  the  extent  of  0.05  per  cent,  is  also  present. 

Mechanism  of  Secretion. — During  the  time  of  lactation  the  mammary 
gland  exhibits  periods  of  secretory  activity  which  alternate  with  periods  of 
rest.  Coincidently  with  these  periods,  certain  histologic  changes  take 
place  in  the  secreting  structures  of  the  gland.  At  the  close  of  a  period  of 
active  secretion  each  acinus  presents  the  following  features :  the  epithelial 
cells  are  short,  cubic,  nucleated,  and  border  a  relatively  wide  lumen  in 
which  is  to  be  found  a  variable  quantity  of  non-discharged  milk.  After 
the  gland  has  rested  for  some  time,  active  metabolism  again  begins.  The 
epithelial  cells  grow  and  elongate;  the  nucleus  divides  into  two  or  three 
new  nuclei,  and  at  the  same  time  the  cell  becomes  constricted;  the  inner 
portion  is  detached  and  is  discharged  into  the  lumen,  Coinciden tally  with 
these  changes  oil-globules  make  their  appearance  in  the  cell  protoplasm, 
some  of  which  are  discharged  separately  into  the  lumen,  while  others 
remain  for  a  time  associated  with  the  detached  cell.  From  these  his- 
tologic changes  it  would  appear  that  the  caseinogen  and  the  fat-globules 
are  metabolic  products  of  the  cell  protoplasm,  and  not  derived  directly 
from  the  blood.  That  lactose  has  a  similar  origin  appears  certain  from 
the  fact  that  it  is  formed  independently  of  carbohydrate  food.  The  water 
and  inorganic  salts  are  doubtless  secreted  by  a  mechanism  similar  to  that 
of  all  other  secreting  glands. 

Colostrum. — Within  a  day  or  two  after  parturition  the  alveoli  become 
filled  with  a  fluid  which  in  some  respects  resembles  milk  and  which  has 
been  termed  colostrum.  This  is  a  watery  fluid  containing  disintegrated 
epithelial  cells  and  fat-globules,  as  well  as  colostrum  corpuscles,  which 
are  probably  leukocytes  containing  fine  fat-globules.  Colostrum  is 
distinguished  from  milk  in  being  richer  in  sugar  and  inorganic  salts.  It 
also  differs  from  milk  in  undergoing  coagulation  by  heat  which  is  supposed 
to  be  due  to  the  presence  of  a  globulin.  Its  coagulation  point  is  about 
72°C.  It  is  said  to  possess  constituents  which  act  as  a  laxative  to  the 
young  child. 


158  HUMAN  PHYSIOLOGY 

LIVER 

The  liver  is  a  highly  vascular,  conglomerate  gland,  appended  to  the 
alimentary  canal.  It  is  the  largest  gland  in  the  body,  weighing  about  four 
and  one-half  pounds;  it  is  situated  in  the  right  hypochondriac  region,  and  is 
retained  in  position  by  five  ligaments,  four  of  which  are  formed  by  dupli- 
catures  of  the  peritoneal  investment. 

The  proper  coat  of  the  liver  is  a  thin  but  firm  fibrous  membrane,  closely 
adherent  to  the  surface  of  the  organ,  which  it  penetrates  at  the  transverse 
fissure,  and  follows  the  vessels  in  their  ramifications  through  its  substance, 
constituting  Glisson^s  capsule. 

Structure  of  the  Liver. — The  liver  is  made  up  of  a  large  number  of  small 
bodies  (the  lobules) ^  rounded  or  ovoid  in  shape,  measuring  J^s  of  an  inch 
in  diameter,  separated  by  a  space  in  which  are  situated  blood-vessels, 
nerves,  hepatic  ducts,  and  lymphatics. 

The  lobules  are  composed  of  cells,  which,  when  examined  microscopic- 
ally, exhibit  a  rounded  or  polygonal  shape,  and  measure,  on  the  average, 
Ho 00  of  3,n  inch  in  diameter;  they  possess  one,  and  sometimes  two,  nuclei; 
they  also  contain  globules  of  fat,  pigment  matter,  and  animal  starch. 
The  cells  constitute  the  secreting  structure  of  the  liver,  and  are  the  true 
hepatic  cells. 

The  Blood-vessels. — The  blood-vessels  which  enter  the  liver  are: 

1,  The  portal  vein,  made  up  of  the  gastric,  splenic,  and  superior  and 
inferior  mesenteric  veins. 

2.  The  hepatic  artery,  a  branch  of  the  celiac  axis. 

Both  the  portal  vein  and  the  hepatic  artery  are  invested  by  a  sheath  of 
areolar  tissue. 

The  vessels  which  leave  the  liver  are  the  hepatic  veins,  originating  in  its 
interior,  collecting  the  blood  distributed  by  the  portal  vein  and  hepatic 
artery,  and  conducting  it  to  the  ascending  vena  cava. 

Distribution  of  Vessels.- — The  portal  vein  and  the  hepatic  artery,  upon  en- 
tering the  liver,  penetrate  its  substance,  divide  into  smaller  and  smaller 
branches,  occupy  the  spaces  between  the  lobules,  completely  surrounding 
and  limiting  them,  and  constitute  the  interlobular  vessels.  The  hepatic 
artery,  in  its  course,  gives  off  branches  to  the  walls  of  the  portal  vein  and 
Glisson's  capsule,  and  finally  empties  into  the  small  branches  of  the  portal 
vein  in  the  interlobular  spaces. 

The  interlobular  vessels  form  a  rich  plexus  around  the  lobules,  from  which 
branches  pass  to  neighboring  lobules  and  enter  their  substance,  where  they 


LIVER  159 

form  a  very  fine  net  work  of  capillary  vessels,  ramifying  over  the  hepatic 
cells,  in  which  the  various  functions  of  the  liver  are  performed.  The  blood 
is  then  collected  by  small  veins,  converging  toward  the  center  of  the 
lobule,  to  form  the  intralobular  vein,  which  runs  through  its  long  axis 
and  empties  into  the  suUohular  vein.  The  hepatic  veins  are  formed  by 
the  union  of  the  sublobular  veins,  and  carry  the  blood  to  the  ascending 
vena  cava;  their  walls  are  thin  and  adherent  to  the  substance  of  the  hepatic 
tissue. 

Bile  Capillaries  and  Hepatic  Ducts. — The  bile  capillaries  are  narrow 
channels  which  penetrate  the  lobule  in  all  directions  and  are  generally 
found  running  along  the  sides  of  the  cells.  These  channels,  which  are 
devoid  of  walls,  receive  from  the  cells  some  of  the  products  of  their  secretor 
activity,  and  hence  are  comparable  to  the  lumen  of  the  alveoli  of  other 
secreting  glands.  At  the  periphery  of  the  lobules  the  bile  capillaries 
communicate  with  large  channels  which  are  the  beginnings  of  the  hepatic 
or  bile-ducts  lying  in  the  interlobular  spaces.  The  interlobular  bile-ducts 
possess  a  distinct  wall  lined  by  flattened  epithelium.  There  is,  however, 
no  distinct  line  of  demarcation  between  the  cells  of  the  interlobular  ducts 
and  the  secreting  cells  of  the  liver  proper,  as  the  two  blend  insensibly,  the 
one  into  the  other.  As  the  hepatic  ducts  increase  in  size  they  gradually 
acquire  the  structure  characteristic  of  the  main  hepatic  duct:  viz.,  a 
mucous,  a  muscle,  and  a  fibrous  coat.  Two  ducts  emerge  from  the  liver 
which  after  a  short  course  unite  to  form  the  main  hepatic  duct.  The 
main  hepatic  emerges  from  the  liver  at  the  transverse  fissure.  At  a  dis- 
tance of  about  5  centimeters  it  is  joined  by  the  cystic  duct,  the  distal 
extremity  of  which  expands  into  a  pear-shaped  reservoir — the  gall-bladder 
in  which  a  portion  of  the  bile  is  temporarily  stored.  The  duct  formed  by 
the  union  of  the  hepatic  and  cystic  ducts — the  common  bile  duct  passes 
forward  for  a  distance  of  about  7  centimeters  and  opens  into  the 
duodenum. 

Functions  of  the  Liver. — The  liver  is  a  complex  organ  having  a  variety 
of  relations  to  the  general  processes  of  the  body.  While  its  physiologic* 
actions  are  not  yet  wholly  understood,  it  may  be  said  that  it  is  engaged : 

1.  In  the  secretion  of  bile, 

2.  In  the  production  of  starch  (glycogen)  and  sugar  (glucose). 

3.  In  the  formation  of  urea. 

4.  In  the  conjugation  of  products  of  protein  putrefaction. 

The  Secretion  of  Bile. — The  characteristic  constituents  of  the  bile  do 
not  preexist  in  the  blood,  but  are  formed  in  the  interior  of  the  liver  cells  of 
materials  derived  from  the  venous  and  arterial  blood.     The  hepatic  cells, 


l6o  ^  HUMAN  PHYSIOLOGY 

absorbing  these  materials,  elaborate  them  into  bile-elements,  and  in  so 
doing  undergo  histologic  changes  similar  to  those  exhibited  by  other  secre- 
tory glands.  The  bile  once  formed,  it  passes  into  the  mouths  of  the  bile 
capillaries,  near  the  periphery  of  the  lobules.  Under  the  influence  of  the 
vis  a  tergo  of  the  new-formed  bile  it  flows  from  the  smaller  into  the  large 
bile-ducts,  and  finally  empties  into  the  intestine,  or  is  regurgitated  into  the 
gall-bladder,  where  it  is  stored  up  until  it  is  required  for  the  digestive  proc- 
ess in  the  small  intestine.  The  study  of  the  secretion  of  bile  by  means  of 
biliary  fistulae  reveals  the  fact  that  the  secretion  is  continuous  and  not 
intermittent;  that  the  hepatic  cells  are  constantly  pouring  bile  into  the 
ducts,  which  convey  it  into  the  gall-bladder.  As  this  fluid  is  required 
only  during  intestinal  digestion,  it  is  only  then  that  the  walls  of  the  gall- 
bladder contract  and  discharge  it  into  the  intestine. 

The  flow  of  bile  from  the  liver  cells  into  the  gall-bladder  is  accomplished 
by  the  inspiratory  movements  of  the  diaphragm,  and  by  the  contraction  of 
the  muscle-fibers  of  the  biliary  ducts,  as  well  as  the  pressure  of  new-formed 
bile.  Any  obstacle  to  the  outflow  of  bile  into  the  intestine  leads  to  an 
accumulation  within  the  bile-ducts.  The  pressure  within  the  ducts 
increasing  beyond  that  of  the  blood  within  the  capillaries,  a  reabsorption 
of  biliary  matters  by  the  lymphatics  takes  place,  giving  rise  to  the  phenom- 
ena of  jaundice. 

The  bile  is  both  a  secretion  and  an  excretion;  it  contains  new  constituents, 
which  are  formed  only  in  the  substance  of  the  liver,  and  are  destined  to  play 
an  important  part  ultimately  in  nutrition;  it  contains  also  waste  ingred- 
ients, which  are  discharged  into  the  intestinal  canal  and  eliminated  from 
the  body. 

The  Production  of  Glycogen  and  Sugar. — In  addition  to  the  preceding 
function,  Bernard,  in  1848,  demonstrated  the  fact  that  the  liver,  during 
life,  normally  produces  a  substance  analogous  in  its  chemic  composition 
to  starch,  which  he  termed  glycogen;  also  that,  when  the  liver  is  removed 
from  the  body,  and  its  blood-vessels  are  thoroughly  washed  out,  after  a 
few  hours  sugar  makes  its  appearance  in  abundance.  The  sugar  can  also 
be  shown  to  exist  in  the  blood  of  the  hepatic  vein  as  well  as  in  a  decoction 
of  the  liver  substance  by  means  of  either  Trommer's  or  Fehling's  test, 
even  when  the  blood  of  the  portal  vein  does  not  contain  a  trace  of  sugar. 

Origin  and  Destination  of  Glycogen. — Glycogen  appears  to  be  formed 
in  the  liver  cells,  from  materials  derived  from  the  food,  whether  the  diet 
be  animal  or  vegetable,  though  a  larger  percentage  is  formed  when  the 
animal  is  fed  on  starchy  and  saccharine  than  when  fed  on  animal  food. 
The  dextrose,  which  is  one  of  the  products  of  digestion,  is  absorbed  by  the 


LIVER  l6l 

blood-vessels  and  carried  directly  into  the  liver;  as  it  does  not  appear  in 
the  urine,  as  it  would  if  injected  at  once  into  the  general  circulation,  it  is 
probable  that  it  is  detained  in  the  liver,  dehydrated,  and  stored  up  as 
glycegen.     The  change  is  shown  by  the  following  formula: 

C6H12O3  -  H2O  =  CsHioOs. 

Dextrose.       Water.     Glycogen. 

The  glycogen  thus  formed  is  stored  up  in  the  hepatic  cells  for  the  future 
requirements  of  the  system.  When  sugar  is  needed  for  nutritive  purposes, 
the  glycogen  is  transformed  into  dextrose  by  the  agency  of  a  ferment. 

Glycogen,  when  obtained  from  the  liver,  is  an  amorphous,  starch-like 
substance,  of  a  white  color,  tasteless  and  colorless,  and  soluble  in  water; 
by  boiling  with  dilute  acids,  or  subjected  to  the  action  of  an  animal  fer- 
ment, it  is  easily  converted  into  dextrose.  When  an  excess  of  sugar  is 
generated  by  the  liver  out  of  the  glycogen,  dextrose  can  be  found  not  only 
in  the  blood  of  the  hepatic  vein,  but  also  in  other  portions  of  the  vascular 
apparatus. 

The  Formation  of  Urea. — The  liver  is  now  regarded  by  many  physiolo- 
gists as  the  principal  organ  concerned  in  urea  formation. 

The  antecedent  of  the  urea,  the  substances  out  of  which  the  liver  cells 
form  urea,  are  for  the  most  part  the  ammonium  salts,  the  carbonate  and 
carbamate,  which  are  brought  to  the  liver  by  the  blood  of  the  portal  vein. 
These  salts  are  formed  largely  in  the  intestinal  wall  out  of  the  amino  acids 
that  result  from  the  digestion  of  proteins.  It  is  also  very  probable  that 
they  arise  from  the  disintegration  of  amino-acids  in  other  portions  of 
the  body. 

The  Conjugation  of  Products  of  Protein  Putrefaction. — One  of  the 

important  functions  of  the  liver  is  the  conversion  of  toxic  compounds,  the 
products  of  the  putrefaction  of  proteins,  into  non-toxic  compounds. 
These  compounds  are  formed  in  the  intestine,  are  absorbed  and  carried  by 
the  blood  of  the  portal  vein  to  the  liver.  In  their  passage  through  the 
capillaries  of  the  liver  they  are  conjugated  for  the  most  part  with  potas- 
sium sulphate  by  the  action  of  the  liver  cells  and  thus  deprived  of  their 
toxicity.  Among  the  substances  thus  conjugated  are  indol,  skatol,  phenol, 
and  cresol.  After  absorption  indol  and  skatol  are  oxidized  to  indoxyl 
and  skatoxyl  and  then  combined  with  potassium  sulphate  giving  rise  to 
potassium  indoxyl  sulphate  and  potassium  skatoxyl  sulphate.  Phenol 
and  cresol  are  apparently  directly  combined  with  potassium  sulphate. 
All  of  these  compounds  then  pass  into  the  blood  of  the  general  circulation 
and  finally  are  eliminated  by  the  kidneys.  Potassiupi  indoxyl  sulphate 
or  indican  is  the  source  of  the  indigo-forming  substance  found  in  the 
11 


1 62  HUMAN  PHYSIOLOGY 

urine.  Other  compounds  are  like-wise  reduced  in  toxicity  by  the  liver 
cells  though  the  methods  by  which  this  is  accomplished  vary  with  the 
nature  of  the  compound.  The  liver  thus  presents  a  chemic  defense  against 
the  entrance  of  more  or  less  toxic  agents  into  the  blood  of  the  general 
circulation. 

INTERNAL  SECRETIONS 

An  internal  secretion  may  be  defined  as  a  more  or  less  complex  material 
or  agent,  produced  by  the  secretor  activities  of  epithelial  cells  of  organs 
and  tissues,  and  which  are  discharged  into  the  blood  and  distributed  to 
organs  more  or  less  remote,  the  activities  of  which  they  influence  in 
varying  ways  and  degrees.  Some  increase,  some  inhibit  physiologic 
processes  while  others  stimulate  growth  and  in  different  ways  modify 
metabolism.  The  internal  secretion  in  many,  if  not  all  instances  belongs 
to  a  class  of  agents  known  as  hormones^  agents  of  known  or  unknown 
composition,  characterized  by  a  relatively  simple  chemic  or  molecular 
composition,  an  easy  diffusibility  across  the  walls  of  the  capillary  blood- 
vessels, a  ready  susceptibility  to  oxidation  and  a  rapid  elimination,  as  a 
result  of  which,  their  action  does  not  continue  indefinitely. 

Glands  of  Internal  Secretion  or  Endocrinous  Glands. — The  glands  con- 
sist mainly  of  epithelial  cells  in  close  relation  to  the  walls  of  capillary 
blood-vessels  and  lymphatics,  and  in  some  instances,  if  not  all,  under  the 
control  of  the  central  nerve  system.  By  reason  of  the  absence  of  ducts 
and  their  relation  to  blood-vessels  they  have  also  been  termed  ductless 
glands  and  vascular  glands  and  inasmuch  as  the  secretion  is  discharged 
internally  (into  the  blood)  they  have  been  designated  endocrinous  glands. 

The  glands  which  fall  into  this  category  are  the  thyroid,  the  parathy 
roids,  the  adrenals,  the  hypophysis  cerebri  or  pituitary,  the  pancreas, 
the  ovaries  and  testicles. 

Thyrbid  Gland. — The  thyroid  gland  or  body  consists  of  two  lobes 
situated  on  the  lateral  aspect  of  the  upper  part  of  the  trachea.  Each 
lobe  is  pyriform  in  shape,  the  base  directed  downward  and  on  a  level 
with  the  fifth  or  sixth  tracheal  ring.  The  lobe  is  about  50  mm.  in  length, 
20  mm.  in  breadth,  and  25  mm.  in  thickness.  As  a  rule,  the  lobes  are 
united  by  a  narrow  band  or  isthmus  of  the  same  tissue.  In  color  the 
gland  is  reddish,  and  it  is  abundantly  supplied  with  blood-vessels  and 
lymphatics. 

Microscopic  examination  shows  that  the  thyroid  consists  of  an  enormous 
number  of  closed  sacs  or  vesicles,  variable  in  size,  the  largest  not  measur- 
ing more  than  o.i  mm.  in  diameter.     Each  sac  is  composed  of  a  thin 


INTERNAL   SECRETIONS  163 

homogeneous  membrane  lined  by  cuboid  epithelium.  The  mterior  of 
the  sac  in  adult  life  contains  a  transparent  viscid  fluid,  containing  albumin 
and  termed  ''colloid"  substance.  Externally,  the  sacs  are  surrounded  by 
a  plexus  of  capillary  blood-vessels  and  lymphatics.  The  individual  sacs 
are  united  and  supported  by  connective  tissue,  which  forms,  in  addition,  a 
covering  for  the  entire  gland.  The  knowledge  at  present  possessed  as  to 
the  function  of  the  thyroid  gland,  especially  in  mammals,  is  the  outcome  of 
a  study  of  the  effects  which  follow  its  arrest  of  development  in  the  child, 
its  degeneration  in  the  adult,  its  extirpation  in  the  human  being  as  well 
as  in  animals. 

Congenital  absence  of  the  gland  or  its  arrested  development  in  early 
childhood  is  followed  by  a  defective  physical  and  mental  development 
characterized  by  the  group  of  phenomena  termed  cretinism. 

Degenerative  processes  which  arise  in  the  thyroid  in  the  adult  give  rise 
to  a  group  of  phenomena  to  which  the  term  myxedema  has  been  given. 
The  most  striking  of  these  phenomena  is  a  swollen  condition  of  the  skin, 
the  result  of  hyperplasia  of  the  subcutaneous  connective  tissue  of  an 
embryonic  type,  rich  in  mucin.  Partly  in  consequence  of  this  change  in 
the  skin  the  face  becomes  broader,  swollen  and  flattened  with  a  loss  of 
expression.  With  the  progress  of  the  degeneration,  the  mind  becomes 
dull  and  clouded,  the  memory  defective  and  finally  the  condition  of 
idiocy  may  be  established. 

Surgical  removal  of  the  thyroid  when  complete,  for  relief  from  symptoms 
due  to  grave  pathologic  changes,  has  been  followed  in  human  beings  by 
symptoms  similar,  if  not  identical  with  those  of  myxedema.  To  this 
condition  the  terms  operative  myxedema  and  cachexia  strumipriva  have 
been  applied.  Removal  of  the  gland  from  animals  is  followed  by  the  same 
symptoms  and  death  in  from  two  to  three  weeks.  From  these  facts  it  is 
evident  that  the  presence  of  the  thyroid  is  essential  to  the  normal  activity 
of  the  tissues  generally.  As  to  the  manner  in  which  it  exerts  its  favorable 
influence,  there  is  some  difference  of  opinion.  The  view  that  the  gland 
removes  from  the  blood  certain  toxic  bodies,  rendering  them  innocuous, 
and  thus  preserving  the  body  from  a  species  of  auto-intoxication,  is  grad- 
ually yielding  to  the  more  probable  view  that  the  epithelium  is  engaged  in 
the  secretion  of  a  specific  material,  which  finds  its  way  into  the  blood  or 
lymph  and  in  some  unknown  way  influences  favorably  tissue  metabolism. 
This  view  of  the  function  of  the  thyroid  is  supported  by  the  fact  that  suc- 
cessful grafting  of  a  portion  of  the  thyroid  beneath  the  skin  or  in  the 
abdominal  cavity  will  prevent  the  usual  symptoms  which  follow  thyroid- 
ectomy. The  same  result  is  obtained  by  the  intravenous  injection  of 
thyroid  juice  or  by  the  administration  of  the  raw  gland.     The  retention 


1 64  HUMAN   PHYSIOLOGY 

of  a  smalt  portion  of  the  gland  when  it  is  removed  by  surgical  means 
will  prevent  the  occurrence  of  operative  myedema. 

Hyperthyroidism,  a  condition  characterized  by  vertigo,  increased  cardiac 
action,  flushing,  tremors,  glycosuria,  and  in  monkeys,  exophthalmos  and  a 
widening  of  the  palpebral  fissure,  may  be  developed  by  the  administration 
of  large  doses  of  the  gland  extracts.  From  these  facts  the  inference  has 
been  drawn  from  the  clinical  side  that  the  symptoms  comprised  under  the 
term  exophthalmic  goiter,  viz. :  rapid  action  of  the  heart,  pulsation  of  the 
large  arteries  at  the  base  of  the  neck,  protrusion  of  the  eyeballs  and  fine 
tremors  of  the  hands,  are  due  to  an  enlargement  of  the  gland  and  a  hyperse- 
cretion of  the  thyroid  cells,  a  condition  spoken  of  as  hyperthyroidism.  This 
inference  has  apparently  been  confirmed  by  the  disappearance  of  the 
symptoms  after  the  removal  of  a  large  portion  of  the  gland,  care  being 
taken  to  leave  a  small  portion  sufficiently  large,  however,  to  produce  the 
necessary  amount  of  the  internal  secretion. 

The  Thyroid  Secretion. — The  chemic  features  of  the  material  secreted 
and  obtained  from  the  structures  of  the  thyroid  indicate  that  it  is  a  complex 
protein  containing  iodin,  which,  under  the  influence  of  various  reagents, 
undergoes  cleavage,  giving  rise  to  a  non-protein  residue,  which  carries  with  it 
the  iodin  and  phosphorus.  The  amount  of  iodin  in  the  thyroid  varies  from 
0.33  to  I  milligram  for  each  gram  of  tissue.  To  this  compound  the  term 
thyro-iodin  has  been  given.  The  administration  of  this  compound  pro- 
duces effects  similar  to  those  which  follow  the  therapeutic  administration 
of  the  fresh  thyroid  itself,  viz. :  a  diminution  of  all  myxedematous  symp- 
toms. In  normal  states  of  the  body,  thyro-iodin  influences  very  actively 
the  general  metabolism.  It  gives  rise  to  a  decomposition  of  fats  and  pro- 
teins and  to  a  decline  in  body-weight.  In  large  doses  it  may  produce  toxic 
symptoms,  e.g.^  increased  cardiac  action,  vertigo,  and  glycosuria. 

The  Function  of  the  Thyroid  Gland. — The  function  or  the  physiologic 
action  of  the  thyroid  gland  itself  is  to  produce  an  internal  secretion  which 
after  its  entrance  into  the  blood  promotes  favorably  the  metabolism  of  the 
neuro-muscular  systems  at  least.  The  myxedema  and  the  failure  of  the 
mental  powers  are  attributed  to  the  loss  or  degeneration  of  the  gland  and 
hence  its  internal  secretion,  and  cretinism  to  the  arrest  of  its  development. 

The  Parathyroids. — The  parathyroids  are  small  bodies,  usually  four 
in  number,  two  on  each  side.  They  are  divided  into  superior  and  in- 
ferior. The  superior  are  situated  internally  and  on  the  posterior  sur- 
face in  close  relation  to,  and  frequently  embedded  in,  the  substance  of 
the  thyroid;  the  inferior  are  situated  externally,  sometimes  in  contact 
with,  and  at  other  times  removed  a  variable  distance  from  the  thyroid. 


INTERNAL   SECRETIONS  165 

Microscopically  the  parathyroids  consist  of  thick  cords  of  epithelial 
cells  separated  by  septa  of  fine  connective  tissue  and  surrounded  by 
capillary  blood-vessels.  Chemic  analysis  shows  that  they  also  contain 
iodin  in  combination  with  some  organic  compound. 

Effects  of  Parathyroid  Removal. — The  surgical  removal  of  the  para- 
thyroids is  followed  in  the  course  of  from  two  to  five  days  by  the  death  of 
the  animal  preceded  in  most  instances  by  a  series  of  symptoms  which  are 
embraced  under  the  general  term  "tetany."  These  symptoms  are  fibril- 
lary contractions  of  muscles,  tremors,  spasmodic  contractions  and  paraly- 
ses of  groups  of  muscles  and  not  infrequently  convulsive  seizures  and  coma. 
During  the  convulsion  there  is  an  acceleration  of  the  heart-beat,  and  in- 
crease in  the  respiratory  movements  which  frequently  become  dyspneic  in 
character.  There  is  also  a  loss  of  appetite,  nausea,  mucous  vomiting,  and 
diarrhea.  Death  may  occur  during  a  convulsion  or  from  coma.  (Morat 
and  Doyon.) 

These  results  for  the  most  part  occur  only  when  all  the  parathyroids  are 
removed.  It  is  asserted  that  even  if  one  gland  is  retained  the  animal  does 
not  die.  The  above  described  symptoms  may  manifest  themselves,  how- 
ever, but  they  are  slight  in  degree. 

The  Hypophysis  Cerebri. — This  is  a  small  body  lodged  in  the  sella  turcica 
of  the  sphenoid  bone.  It  consists  of  an  anterior  lobe,  somewhat  red  in 
color,  and  a  posterior  lobe,  yellowish-gray  in  color.  The  former  is  much 
the  larger  and  partly  embraces  the  latter.  The  anterior  lobe  is  developed 
from  an  invagination  of  the  epiblast  of  the  mouth  cavity,  and  consists  of 
distinct  gland  tissue.  The  posterior  lobe  is  an  outgrowth  from  the  brain 
and  is  connected  with  the  infundibulum  by  a  short  stalk.  It  has  been 
suggested  that  the  term  infundibular  body  be  reserved  for  the  posterior 
lobe.  This  distinction  appears  to  be  desirable,  inasmuch  as  in  their  origin 
and  structure  they  are  separate  and  distinct  bodies. 

Complete  removal  of  the  hypophysis  cerebri,  or  the  pituitary  body,  is 
always  followed  by  a  fatal  result,  preceded  by  symptoms  not  unlike  those 
which  follow  removal  of  the  thyroid:  viz.,  unsteadiness  of  gait,  muscular 
twitchings,  lethargy,  fall  of  blood  pressure,  lowering  of  the  body  tempera- 
ture, coma  and  death. 

Partial  removal  of  the  anterior  lobe  is  much  less  fatal,  though  adult  animals 
become  adipose  and  degenerate  sexually.  Young  animals  remain  under- 
sized and  fail  to  develop  sexual  characteristics.  Sexual  infantilism  per- 
sists. From  these  and  similar  facts  it  has  been  assumed  that  sexual 
infantilism  is  due  to  defective  activity  of  the  anterior  lobe.    Hyperactivity 


1 66  HUMAN   PHYSIOLOGY 

of  the  anterior  lobe  in  early  life  may  lead  to  giantism  and  in  the  adult  to 
acromegaly. 

Removal  of  the  posterior  lobe  leads  to  an  increased  tolerance  for  and 
assimilation  of  sugar  which  eventually  contributes  to  the  formation  and 
deposition  of  fat.  On  the  contrary  a  hyperactivity  of  the  posterior  lobe 
leads  to  a  diminished  tolerance  for  sugar  as  shown  by  the  appearance  of 
hyperglycemia  and  glycosuria.  The  internal  secretion  of  the  posterior 
lobe  is  believed  to  be  the  hyaline  granules  which,  streaming  through  the 
lobe,  are  discharged  into  the  third  ventricle. 

Intravenous  injection  of  pituitary  extracts  or  the  pharmaceutical  prepa- 
ration pituUrin  is  followed  by  a  rise  of  blood  pressure  from  a  contraction  of 
the  arteriole  muscles,  and  an  inhibition  of  the  heart.  It  also  causes  dila- 
tation of  the  renal  vessels  and  stimulates  specifically  the  renal  cells  to  activ- 
ity, thus  causing  a  marked  diuresis.  The  extract  also  stimulates  the 
non- striated  muscles  of  the  intestines,  bladder,  uterus,  mammary  glands,  as 
well  as  the  dilatator  muscle  of  the  iris. 

The  Functions  of  the  Pituitary  or  Hypophysis. — The  functions  of  the 
pituitary  body  are  related  to  the  activities  of  the  anterior  and  posterior 
lobes.  The  anterior  lobe,  through  its  internal  secretion,  stimulates  the 
growth  of  the  skeleton  and  associated  tissues  as  apparently  shown  by  the 
fact  that  an  excess  of  secretion  in  early  life  leads  to  giantism  and  in  adult 
life  to  acromegaly,  while  a  deficiency  of  secretion  leads  to  defective  growth 
and  the  establishment  of  infantilism.  The  posterior  lobe  through  its  inter- 
nal secretion  assists  in  the  regulation  of  carbohydrate  metabolism  as  shown 
by  the  fact  that  an  excess  of  secretion  lowers  the  assimilation  capacity  and 
thus  develops  glycosuria,  while  a  deficiency  of  the  secretion  raises  the  as- 
similation capacity  and  leads  to  the  production  and  accumulation  of  fat. 

Adrenal  Bodies,  or  Suprarenal  Capsules. — These  are  two  flattened 
bodies,  somewhat  crescentic  or  triangular  in  shape,  situated  each  upon  the 
upper  extremity  of  the  corresponding  kidney,  and  held  in  place  by  con- 
nective tissue.  They  measure  about  40  mm.  in  height,  30  mm.  in  breadth, 
and  from  6  to  8  mm.  in  thickness.     The  weight  of  each  is  about  4  gm. 

Histology. — The  gland  is  covered  externally  by  a  fibrous  tissue  from 
which  septa  pass  into  the  more  central  portions  thus  forming  a  framework 
for  the  support  of  blood-vessels  and  cells. 

A  section  of  the  gland  shows  just  beneath  the  capsule  an  outer  portion 
termed  the  cortex  and  an  inner  portion  termed  the  medulla.  The  cortex 
consists  mainly  of  cuboid  cells  arranged  in  cylindric  columns.  The  outer 
ay^rs  of  cells  are  arranged  in  irregular  masses  forming  what  has  been 


INTERNAL   SECRETIONS  1 67 

called  the  zona  glomerulosa.  The  medulla  consists  of  uniting  and  inter- 
lacing cords  of  polyhedral  cells,  the  cytoplasm  of  which  contains  granular 
matter  and  a  distinct  nucleus.  When  treated  with  chromic  acid  or  chro- 
mium salts  the  cytoplasm  stains  a  dull  brown  or  yellow  color.  For  this 
reason  they  are  termed  chromatin  cells.  Similar  cells  are  found  in  sym- 
pathetic ganglia. 

The  gland  receives  blood  from  branches  of  the  renal  artery;  it  discharges 
its  venous  blood  by  way  of  the  adrenal  veins  into  the  vena  cava  on  the 
right  side  and  the  renal  vein  on  the  left  side.  The  gland  cells  are  excited  to 
activity,  the  central  nerve  system  through  the  splanchnics  and  their  con- 
tinuations, branches  from  the  semi-lunar  ganglion. 

Destructive  pathologic  processes  of  the  adrenals  produce  a  profound  dis- 
turbance of  the  nutrition  first  described  by  Addison  and  subsequently 
termed  by  Trousseau,  Addison's  disease,  which  is  characterized  by 
extreme  muscular  weakness  and  an  incapacity  for  sustained  muscle 
activity;  a  bronze-like  discoloration  of  the  skin  and  mucous  membranes, 
disturbance  of  the  digestive  functions,  indicated  by  indigestion,  vomiting 
and  diarrhea;  a  feeble  action  of  the  heart;  a  small  feeble  pulse;  a  low  blood- 
pressure;  a  subnormal  temperature  and  a  feeble  respiration.  Death 
ensues  from  paralysis  of  the  respiratory  muscles. 

Surgical  removal  of  these  bodies  from  various  animals  is  invariably  and 
in  a  short  time  followed  by  death,  preceded  by  some  of  the  symptoms 
characteristic  of  Addison's  disease.  Their  development,  however,  is  more 
acute.  From  the  fact  that  animals  so  promptly  die  from  extirpation  of 
these  bodies,  and  the  further  fact  that  the  blood  of  such  animals  is  toxic  to 
those  the  subjects  of  recent  extirpation,  but  not  to  normal  animals,  the 
conclusion  was  drawn  that  the  function  of  the  adrenal  bodies  was  to 
remove  from  the  blood  some  toxic  material  the  product  of  muscle  metabo- 
lism. Its  accumulation  after  extirpation  gives  rise  to  death  through 
auto-intoxication. 

The  intravenous  injection  of  adrenal  extracts  is  followed  in  a  very  short 
time  by  a  marked  rise  in  blood  pressure  and  if  the  dose  be  large  enough,  by 
a  cessation  of  the  auricular  beat,  though  the  ventricular  beat  continues 
though  with  a  slower  rhythm.  If  the  vagi  are  cut  previous  to  the  injec- 
tion or  if  the  inhibition  is  removed  by  atropin,  the  rapidity  and  vigor  of 
both  auricles  and  ventricles  are  increased.  Whether  the  inhibitory 
influence  is  removed  or  not,  there  is  a  marked  increase  in  the  blood- 
pressure,  though  it  is  greater  in  the  former  than  in  the  latter  instance. 
This  is  attributed  to  a  direct  stimulation  and  contraction  of  the  muscle - 
fibers  of  the  arterioles  themselves,  and  not  to  vaso-motor  influences,  as  it 
occurs  also  after  division  of  the  cord  and  destruction  of  the  bulb.    The 


1 68  HUMAN  PHYSIOLOGY 

contraction  of  the  arterioles  is  quite  general,  as  shown  by  plethysmo- 
graphic  studies  of  the  limbs,  spleen,  kidney,  etc.  The  arterioles  of  the 
lungs  and  brain  do  not  contract  under  its  influence  to  the  same  extent 
as  do  the  arterioles  in  other  regions  of  the  body,  possibly  for  the  reason  that 
they  are  not  so  abundantly  supplied  with  vaso-motor  nerves.  Applied 
locally  to  the  mucous  membranes,  the  adrenal  extract  produces  contraction 
of  the  blood-vessels  and  pallor. 

The  extract  also  diminishes  the  tonus  of  the  muscle  walls  of  the  intestine 
and  other  viscera.  Injection  of  the  extract  into  the  peritoneal  cavity  or 
into  the  veins  causes  hyperglycemia  and  glycosuria  which  may  last  for 
several  hours.  All  these  effects  follow  an  injection  of  an  extract  of  the 
medulla  only. 

The  internal  secretion  is  represented  by  the  alkaloid  termed  epinephrifi 
or  adrenalin.     This  alkaloid  produces  all  the  effects  of  the  extracts. 

The  nerve  system  influences  the  secretory  activity  of  the  adrenals. 
The  major  emotional  disturbances  increase  by  percentage  of  adrenalin  in 
the  blood  which  in  turn  leads  to  hyperglycemia  and  glycosuria. 

The  Function  of  the  Adrenal  Gland. — The  function  of  the  adrenal  gland, 
at  least  of  the  medullary  portion,  is  to  furnish  an  internal  secretion  which 
serves  apparently  to  maintain  that  degree  of  frequency  and  force  of  the 
heart-beat  and  the  contraction  of  the  arteriole  muscle  necessary  to  main- 
tain the  normal  blood-pressure;  to  inhibit  as  occasion  requires,  the  tonus  of 
muscle  walls  of  various  viscera;  to  cause  a  mobilization  of  sugar  in  the 
blood  when  this  is  necessary,  and  to  increase  in  some  unexplained  way  the 
tonus  and  activity  of  the  skeletal  musculature. 

The  Pancreas. — The  pancreas  though  engaged  in  the  production  of  an 
external  secretion  is  yet,  by  reason  of  the  specialized  group  of  cells,  the 
islands  of  Langerhans,  to  be  regarded  as  an  organ  of  an  internal  secretion 
as  well.  These  islands  it  is  generally  believed  are  engaged  in  the  secretion 
of  an  agent  which  after  entering  the  blood  is  carried  to  the  muscles  where 
it  activates  or  assists  a  glycolytic  enzyme  in  promoting  the  oxidation  of 
sugar;  or  it  may  inhibit  normally  the  stimulating  action  of  adrenalin  on  the 
liver  cells  and  thus  prevent  an  excessive  output  of  sugar  and  the  develop- 
ment of  hyperglycemia.  If  the  entire  pancreas  is  extirpated  and  the  ani- 
mal survive  the  operation,  a  glycosuria  is  soon  established,  followed  by  a 
series  of  symptoms  resembling  those  observed  in  diabetes  mellitus  as  it 
occurs  in  man,  viz. :  thirst,  polyuria,  loss  of  energy,  decline  in  body-weight, 
etc.,  followed  by  death  in  a  few  weeks.  Pathologic  processes  that  involve 
a  large  portion  of  the  pancreas  likewise  give  rise  to  a  similar  series  of 


INTERNAL   SECRETIONS  169 

phenomena,  as  ligation  of  the  pancreatic  duct,  a  procedure  that  leads  to  a 
destruction  of  all  portions  of  the  pancreas  except  the  islands  of  Langerhans 
and  without  developing  glycosuria  has  led  to  the  inference  that  these 
islands  are  the  agents  engaged  in  the  production  of  the  internal  secretion. 

The  Testicles  and  Ovaries. — The  testicles  and  ovaries  are  regarded  at 
the  present  time  as  glands  for  the  production  of  an  internal  secretion,  as 
well  as  for  the  production  of  the  characteristic  reproductive  elements. 
The  removal  of  the  testicles  early  in  life  and  before  the  age  of  puberty 
leads  to  imperfect  development  of  the  vesiculae  seminales  and  the  prostate 
gland;  in  addition  to  these  defects,  there  is  a  failure  of  development  of  the 
various  and  distinctly  sexual  characters  peculiar  to  man  and  other  animals 
as  well.  Sexual  desire  is  wanting  and  the  body  frequently  remains  in  the 
infantile  state.  Transplantation  of  the  testicles,  in  cocks  and  in  certain 
of  the  smaller  mammals  that  have  been  castrated,  has  led  to  the  develop 
ment  of  the  secondary  sexual  characters  which  in  no  apparent  way  differed 
from  those  of  control  animals. 

The  ovaries  are  also  regarded  as  glands  for  the  production  of  an  inter- 
nal secretion,  as  well  as  for  the  production  of  characteristic  reproductive 
elements. 

The  removal  of  the  ovaries  of  human  beings  early  in  life  is  an  operation 
that  is  not  often  performed  and  hence  it  is  difficult  to  state  the  results  that 
might  arise.  Their  removal  in  certain  animals  leads  to  an  atrophy  of  the 
uterus,  and  in  addition,  to  a  failure  of  development  of  secondary  sexual 
characters.  Menstruation  does  not  occur  and  the  body  does  not  reach 
maturity.  The  removal  of  the  ovaries  in  adult  life  results  in  a  cessation 
of  menstruation,  and  the  appearance  of  a  variety  of  disorders  of  a  bodily 
and  mental  character.  Similar  phenomena  are  frequently  observed  at  the 
menopause,  when  the  ovaries  undergo  degenerative  changes.  The  admin- 
istration of  extracts  of  the  ovaries — oophorin  tablets — is  claimed  to  relieve 
some  of  the  symptoms  following  the  removal  of  the  ovaries  or  occurring 
during  the  menopause.  The  transplantation  of  an  ovary  into  the  wall  of 
the  uterus  or  into  the  broad  ligament  after  ovariotomy  in  women  has, 
even  after  the  lapse  of  two  years,  reestablished  menstruation  and 
wakened  sexual  desire. 

The  Spleen. — Though  a  ductless  gland  it  can  hardly  be  said  that  the 
spleen  is  a  gland  of  internal  secretion  inasmuch  as  no  experimental  pro- 
cedure supports  such  a  view.  Notwithstanding  all  the  experiments 
which  have  been  made  to  determine  the  functions  of  the  spleen,  it  can 
not  be  said  that  any  very  definite  results  have  been  obtained.    The  fact 


170  HUMAN   PHYSIOLOGY 

that  the  spleen  can  be  removed  from  the  body  of  an  animal  without 
appreciably  interfering  with  the  normal  metabolism  would  indicate  that 
its  function  is  not  very  important.  The  chief  changes  observed  after 
such  a  procedure  are  an  enlargement  of  the  lymphatic  glands  and  an 
increase  in  the  activity  of  the  red  marrow  of  the  bones.  The  presence 
of  large  numbers  of  leukocytes  in  the  splenic  pulp  and  in  the  blood  of 
the  splenic  vein  suggested  the  idea  that  the  spleen  is  engaged  in  the 
production  of  leukocytes,  and  to  this  extent  contributes  to  the  forma- 
tion of  blood.  The  presence  of  disintegrated  red  blood-corpuscles  has 
suggested  the  view  that  the  spleen  exerts  a  destructive  action  on  func- 
tionally useless  red  corpuscles.  These  and  other  theories  as  to  splenic 
functions  have  been  offered  by  different  observers,  but  all  are  lacking 
positive  confirmation. 

Plethysmographic  studies  show,  that  the  splenic  volume  increases  and 
decreases  in  response  to  the  rise  and  fall  of  blood  pressure.  In  addition 
to  these  rhythmic  variations  the  spleen  steadily  increases  in  volume  for 
a  period  of  five  hours  after  digestion,  and  then  steadily  declines  and 
returns  to  its  former  condition. 


THE    CENTRAL    AND    PERIPHERAL    ORGANS    OF    THE 
NERVE  SYSTEM 

All  the  neurons  that  collectively  constitute  the  nerve  system  are  grouped 
into  more  or  less  completely  organized  masses  termed  organs  which  in 
accordance  with  their  location  may  be  divided  into  (i)  central  organs  and 
(2)  peripheral  organs. 

The  Central  Organs. — The  central  organs  of  the  nerve  system  are  the 
encephalon  and  the  spinal  cord,  lodged  within  the  cavity  of  the  cranium 
and  the  cavity  of  the  spinal  column  respectively.  The  general  shape  of 
these  two  portions  of  the  nerve  system  correspond  with  that  of  the  cavities 
in  which  they  are  contained.  The  encephalon  is  broad  and  ovoid,  the 
spinal  cord  is  narrow  and  elongated. 

The  encephalon  is  subdivided  by  deep  fissures  into  four  distinct,  though 
closely  related  portions:  viz.,  (i)  the  cerebrum,  the  large  ovoid  mass  oc- 
cupying the  entire  upper  part  of  the  cranial  cavity;  (2)  the  cerebellum, 
the  wedge-shaped  portion  placed  beneath  the  posterior  part  of  the  cere- 
brum and  lodged  within  the  cerebellar  fossae;  (3)  the  isthmus  of  the 
encephalon,  the  more  or  less  pyramidal-shaped  portion  connecting  the 
cerebrum  and  cerebellum  with  each  other  and  both  with  (4)  the  medulla 
oblongata. 


THE  ORGANS  OF  THE  NERVE  SYSTEM         17I 

The  spinal  cord  is  narrow  and  cylindric  in  shape.  It  occupies  the  spinal 
canal  as  far  down  as  the  second  or  third  lumbar  vertebra. 

The  central  organs  of  the  nerve  system  are  bilaterally  symmetric,  con- 
sisting of  distinct  halves  united  in  the  median  line.  The  cerebrum  is  sub- 
divided by  a  deep  fissure,  running  antero-posteriorly,  into  two  ovoid 
masses  termed  cerebral  hemispheres,'  the  cerebellum  is  also  partially  sub- 
divided into  hemispheres;  the  isthmus  likewise  presents  in  the  median 
line  a  partial  division  into  halves;  the  medulla  oblongata  and  spinal  cord 
are  subdivided  by  an  anterior  or  ventral  and  a  posterior  or  dorsal  fissure 
into  halves,  a  right  and  a  left. 

The  Peripheral  Organs. — The  peripheral  organs  of  the  nerve  system 
in  anatomic  and  physiologic  relation  with  the  central  organs,  are  the  en- 
cephalic and  the  spinal  nerves. 

The  encephalic  nerves^  twelve  in  number  on  each  side  of  the  median  line, 
are  in  anatomic  relation  with  the  base  of  the  encephalon,  and  because  of  the 
fact  that  they  pass  through  foramina  in  the  walls  of  the  cranium  they  are 
usually  termed  cranial  nerves. 

The  spinal  nerves,  thirty-one  in  number  on  each  side,  are  in  anatomic 
relation  with  the  spinal  cord,  and  because  of  the  fact  that  they  pass  through 
foramina  in  the  walls  of  the  spinal  column  they  are  termed  spinal  nerves. 
As  both  cranial  and  spinal  nerves  are  ultimately  distributed  to  the  struc- 
tures of  the  body — i.e.,  the  general  periphery — they  collectively  constitute 
the  peripheral  organs  of  the  nerve  system. 

The  spinal  nerves  consist  of  two  groups  of  nerve-fibers,  a  ventral  and  a 
dorsal  group.  Though  closely  intermingled  in  the  common  trunk  of  the 
spinal  nerve  they  are  distinctly  separated  near  the  spinal  cord.  Owing  to 
their  connection  with  the  ventral  and  dorsal  surfaces  of  the  spinal  cord 
they  have  been  termed  respectively  the  ventral  and  dorsal  roots.  Pe- 
ripherally the  ventral  root  fibers  are  distributed  to  skeletal  muscles,  glands, 
walls  of  blood-vessels  and  walls  of  various  viscera:  the  dorsal  root  fibers 
are  distributed  to  skin,  mucous  membranes,  muscles,  joints,  etc. 

The  relation  of  the  ventral  and  dorsal  roots  of  the  spinal  nerves  to  the 
spinal  cord,  the  classification  of  their  contained  nerve  fibers  and  their 
various  functions  have  been  considered  in  a  previous  section  (see  pages  42, 
44). 

The  encephalic  nerves  also  consist  of  afferent  and  efferent  nerve 
fibers  which  pass  for  the  most  part  to  their  destinations  as  separate  and 
independent  nerves.  Their  relation  to  the  encephalon  and  the  phenomena 
that  follow  their  stimulation  and  division  and  the  functions  attributed 
to  them  will  be  fully  considered  in  a  subsequent  section. 


172  HUMAN   PHYSIOLOGY 

SPINAL  CORD 

The  spinal  cord  varies  from  lo  to  45  cm.,  in  length;  is  12  mm.  in 
thickness,  weighs  42  grams  and  extends  from  the  atlas  to  the  second 
lumbar  vertebra,  terminating  in  the  filum  ferminale.  It  is  cylindric  in 
shape,  and  presents  an  enlargement  in  the  lower  cervical  and  lower  dorsal 
regions,  corresponding  to  the  origin  of  the  nerves  which  are  distributed 
to  the  upper  and  lower  extremities.  The  cord  is  divided  into  two  lateral 
halves  by  the  anterior  and  posterior  fissures.  It  is  composed  of  both 
white  or  fibrous  and  gray  or  vesicular  matter,  the  former  occupying  the 
exterior  of  the  cord,  the  latter  the  interior,  where  it  is  arranged  in  the 
form  of  two  crescents,  one  in  each  lateral  half,  united  by  the  central  mass, 
the  gray  commissure;  the  white  matter  being  united  in  front  by  the  white 
commissure. 

Segmentation  of  the  Spinal  Cord. — For  the  elucidation  of  many  prob- 
lems connected  with  the  physiologic  actions  of  the  spinal  cord,  as  well  as  of 
the  symptoms  which  follow  its  pathologic  impairment,  it  will  be  found 
helpful  to  consider  the  cord  as  consisting  physiologically  of  a  series  of  seg- 
ments placed  one  above  the  other,  the  number  of  segments  correspond- 
ing to  the  number  of  spinal  nerves.  Each  spinal  segment  would  therefore 
comprise  that  portion  of  the  cord  to  which  is  attached  a  pair  of  spinal 
nerves.  The  nerve-cells  in  each  segment  are  in  histologic  and  physio- 
logic relation  with  definite  areas  of  the  body,  embracing  muscles,  blood- 
vessels, glands,  skin,  etc. 

If  the  exact  distribution  of  the  nerves  of  any  segment  were  then  known, 
its  function  could  be  readily  stated.  By  virtue  of  this  segmentation  it 
becomes  possible  for  each  segment  to  act  independently,  or  in  cooperation 
with  other  segments,  near  or  remote,  with  which  they  are  associated  by  the 
intrinsic  or  associative  cells  and  their  axons;  and  the  spinal  cord  itself  is 
enabled  to  act  as  a  unit. 

Structure  of  the  Gray  Matter. — The  gray  matter  is  arranged  in  the  forms 
of  two  crescents,  united  by  a  commissural  band,  forming  a  figure  resem- 
bling the  letter  H.  Each  crescent  presents  a  ventral  and  a  dorsal  horn. 
The  center  of  the  commissure  presents  a  canal  which  extends  from  the 
fourth  ventricle  downward  to  the  filum  terminale.  The  ventral  horn 
is  short  and  broad  and  does  not  extend  to  the  surface.  The  dorsal  horn 
is  narrow  and  elongated  and  extends  quite  to  the  surface.  It  is  covered 
and  capped  by  the  substantia  gelatinosa.  The  gray  matter  consists  pri- 
marily of  a  framework  of  fine  connective  tissue,  supporting  blood-vessels, 


SPINAL   CORD  173 

lymphatics,  meduUated  and  non-medullated  nerve-fibers,  and  groups  of 
nerve-cells. 

Nerve-Cells. — The  nerve-cells  are  arranged  in  groups,  which  extend  for 
some  distance  throughout  the  cord,  forming  columns  more  or  less  continu- 
ous. The  first  group  is  situated  in  the  ventral  horn,  the  cells  of  which 
are  large,  multipolar,  and  connected  with  the  ventral  roots  of  the  spinal 
nerves,  and  are  supposed  to  be  motor  in  function.  The  second  group  is 
situated  in  the  dorsal  horn,  the  cells  of  which  are  spindle-shaped,  and  from 
their  relation  to  the  posterior  roots  are  supposed  to  be  sensory  in  function. 
The  third  group  is  situated  in  the  lateral  aspect  of  the  gray  matter,  and  is 
^  quite  separate  and  distinct,  except 

\\      ;  in  the  lumbar  and  cervical  enlarge- 

■f  %   •       #  ments,  where  it  blends  with  those 

^■-     /''^^^^Rlfi^^X  f       ^^    ^^^    ventral   horn.    A   fourth 

/      ^/BHBk\^     \    -  ■"*        group  is  situated  at  the  inner  base 
L    ^J^^^BH^KV;"    A  ^^  ^^^  dorsal  horn;  it  begins  about 

m0^\  B^^Sb  /^^B  ^^^  seventh  or  eighth  cervical  nerve 

h, ^^H^OTilffiS^^^W  ^^^    extends    downward    to    the 

^^H^SBiiMP^^^  second  or  third  lumbar,  being  most 

^l^^ljl^^^C.  prominent    in    the    dorsal    region. 

;       \    /  hw  This  column  is  known  as  Clark 's 

,  /  C  vesicular  column. 

^            '                         r>  The  nerve  cells  may  be  divided 

Fig.  is. — Scheme  of  the  Conducting  .                                         .... 

Path  in  the  Spinal  Cord  at  the  Third  into   three   groups,   viz. :    intrinsic, 

Dorsal  NERVE.--(La«cfo/5.)  receptive  and  emissive. 

The  black  part  is  the  gray  matter,     v,  ^ 

Ventral,  hw,  dorsal  root,     a.   Direct,  and  -,-         t»    1   x;             ^     xi.       t>          1 

g,g,  crossed,    pyramidal   tracts,    b,    Ven-  J^h©     Relation      Of     the     Dorsal 

trai  funiculus,  ground  bundle,    c,  Goirs  r^q^s  of  the  Spinal  Nerves  to  the 

column,    d,  Postero-extemal  or  Burdach's  \  w»4/*^«*  ^.i^xv^o  w  i^v 

column.  e,e,  and  f,f,  Mixed  lateral  paths.  Intra-Spinal     Cells. — The     nerve 

h,h,  Direct  cerebellar  tracts.  ,.,  •         ^1         1  i  ^ 

fibers   composing  the  dorsal  roots 

may  be  divided  into  two  groups,  viz.;  i.  those  distributed  peripherally  to 

skin  and  mucous  membrane  and  2.  those  distributed  to  tendons,  joints 

and  muscles.     The  fibers  of  the  first  group  receive  their  stimulation  from 

objects  in  the  external  world  and  have  therefore  been  termed  exteroceptive; 

the  fibers  of  the  second  group  receive  their  stimulation  from  changes  taking 

place  in  and  around  the  structures  in  which  they  terminate  and  hence  have 

been  termed  proprioceptive.    The  centrally  directed  fibers  of  the  first  group, 

the  exteroceptive,  on  entering  the  cord  became  related  in  part  either 

directly  or  indirectly  through  an  intercalated  neuron,  with  motor  cells  in 

the  ventral  horn  and  also  in  part  with  afferent  or  receptive  cells  at  the  base 

of  the  dorsal  horn.     The  fibers  of  the  second  group,  the  proprioceptive, 


174  HUMAN  PHYSIOLOGY 

become  related  in  part  to  the  motor  cells  of  the  ventral  horn,  to  the 
cells  composing  Clark's  vesicular  column  and  in  part  after  ascending  the 
dorsal  funiculus  to  the  cells  composing  the  clavate  and  cuneate  nuclei. 

THE  FUNCTIONS  OF  THE  GRAY  MATTER 

The  efferent  cells  of  the  spinal  segments  are  the  immediate  sources  of 
the  nerve  energy  that  excites  activity  in  skeletal  muscles,  glands,  vascu- 
lar, and  to  some  extent  visceral  muscles. 

The  discharge  of  their  energy  many  be  caused : 

1.  By  variations  in  the  composition  of  the  blood  or  lymph  by  which  they 
are  surrounded  or  as  the  outcome  of  a  reaction  between  the  chemic  con- 
stituents of  the  lymph  on  the  one  hand  and  the  chemic  constituents  of  the 
nerve-cell  on  the  other  hand.  The  excitation  of  the  cell  thus  occasioned 
is  termed  automatic  or  autochthonic  excitation. 

2.  By  the  arrival  of  nerve  impulses,  coming  through  afferent  nerves 
from  the  general  periphery,  skin,  mucous  membrane,  etc. 

3.  By  the  arrival  of  nerve  impulses  descending  the  spinal  cord  from 
cells  in  the  cortex  of  the  cerebrum  or  subordinate  regions. 

The  excitation  in  the  former  instances  is  said  to  be  reflex  or  peripheral 
in  origin;  in  the  latter  instance  direct  or  cerebral  in  origin.  In  the  direct 
or  cerebral  excitations  the  skeletal  muscle  movements  are  due  to  volitional, 
the  gland  discharges  and  vascular  and  visceral  muscle  movements  to 
emotional,  phases  of  cerebral  activity. 

Automatic  Excitation. — By  this  expression  is  meant  a  discharge  of 
energy  from  the  spinal  nerve-cells  occasioned  by  (a)  a  change  in  the  chemic 
composition  of  the  blood  and  lymph  by  which  they  are  surrounded  or  prob- 
ably a  reaction  between  the  constituents  of  the  lymph  and  the  constitu- 
ents of  the  nerve-cell  or  (6)  the  developments  within  the  cell  of  a  stimulus, 
the  so-called  '* inner  stimulus,"  the  outcome  of  metabolic  activity. 

As  illustrations  of  such  activity  may  be  mentioned :  {a)  the  contraction 
of  the  abductor  muscle  of  the  larynx  (the  posterior  crico-arytenoid) 
whereby  the  vocal  membranes  are  separated  and  the  glottis  kept  open 
under  all  circumstances  except  during  the  emission  of  vocal  sounds;  {h) 
the  contraction  of  the  dilatator  muscle  of  the  iris;  (c)  the  contraction  of  the 
anal  and  vesic  sphincters;  {d)  the  periodic  contraction  of  the  respiratory 
muscles;  Xe)  the  acceleration  of  the  heart-beat;  (/)  the  more  or  less  con- 
tinuous contraction  of  the  arteriole  muscles  whereby  the  blood-pressure 
is  largely  maintained.  The  nerve  centers  exciting  these  structures  are 
inferred  to  be  in  a  condition  of  continuous  automatic  activity  though 
capable  of  modification  by  nerve  impulses  reflected  to  them  from  more 
or  less  distant  sources. 


THE  FUNCTIONS  OF  THE  GRAY  MATTER        1 75 

Reflex  Excitation. — It  has  already  been  stated  that  the  nerve-cells  in  the 
spinal  cord  are  capable  of  receiving  and  transforming  afferent  nerve 
impulses,  the  result  of  peripheral  stimulation,  into  efferent  nerve  impulses, 
which  are  reflected  outward  to  skeletal  muscles,  exciting  contraction; 
to  glands,  provoking  secretion;  to  blood-vessels,  changing  their  caliber; 
and  to  organs,  inhibiting  or  augmenting  their  activity.  All  such  actions 
taking  place  through  the  spinal  cord  and  medulla  oblongata  independently 
of  sensation  or  volition  are  termed  reflex  actions.  The  mechanism 
involved  in  every  reflex  action  consists  of  at  least  the  following  structures, 
viz.: 

1.  A  receptive  surface;  e.g.j  skin,  mucuous  membrane,  sense  organ, etc. 

2.  An  afferent  fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises — 

4.  An  efferent  nerve,  distributed  to  — 

5.  A  responsive  organ,  as  muscle,  gland,  blood-vessel,  etc. 

If  a  stimulus  of  sufficient  intensity  be  applied  to  the  receptive  surface, 
there  will  be  developed  in  the  terminals  of  the  afferent  nerve  a  series  of 
nerve  impulses  which  will  be  transmitted  by  the  afferent  nerve  to,  and 
received  by,  the  dendrites  of  the  emissive  cell  in  the  anterior  horn  of  the 
gray  matter.  With  the  reception  of  these  impulses  there  will  be  a  disturb- 
ance in  the  equilibrium  of  the  molecules  of  the  cells,  a  liberation  of  energy, 
and  a  transmission  of  nerve  impulses  outward  through  the  efferent  nerve 
to  the  skeletal  muscle,  gland-epithelium,  vascular  or  visceral  muscle. 

In  preceding  sections  many  illustrations  of  reflex  actions  have  been 
presented  in  connection  with  the  consideration  of  the  mechanism  of 
mastication;  the  secretion  of  saliva;  the  muscle,  glandular  and  vascular 
phenomena  of  gastric  and  intestinal  digestion;  the  vascular  and  respiratory 
movements,  the  mechanism  of  micturition,  etc. 

Special  Reflex  Movements. — Among  the  reflexes  connected  with  the 
more  superficial  portions  of  the  body  there  are  some  which  are  so  fre- 
quently either  increased  or  diminished  in  pathologic  conditions  of  the 
spinal  cord  that  their  study  affords  valuable  indications  as  to  the  seat 
and  character  of  the  lesions.     They  may  be  divided  into: 

1.  The  skin  or  superficial  reflexes. 

2.  The  tendon  reflexes. 

3.  The  organ  reflexes. 

The  skin  reflexes,  characterized  by  contraction  of  underlying  muscles 
are  induced  by  stimulation  of  the  afferent  nerve-endings  of  the  skin — e.g, 
by  pricking,  pinching,  scratching,  etc.  The  following  are  the  principal 
skin  reflexes: 


176  HUMAN  PHYSIOLOGY 

1.  Plantar  reflex,  consisting  of  contraction  of  the  muscles  of  the  foot, 
induced  by  stimulation  of  the  sole  of  the  foot;  it  takes  place  through  the 
segments  of  the  cord  which  give  rise  to  the  second  and  third  sacral  nerves. 

2.  Gluteal  reflex^  consisting  of  contraction  of  the  glutei  muscles  when  the 
skin  over  the  buttock  is  stimulated;  it  takes  place  through  the  segments 
giving  origin  to  the  fourth  and  fifth  lumbar  nerves. 

3.  Cremasteric  reflex,  consisting  of  a  contraction  of  the  cremaster 
muscle  and  a  retraction  of  the  testicle  toward  the  abdominal  ring  when  the 
skin  on  the  inner  side  of  the  thigh  is  stimulated;  it  takes  place  through  the 
segments  which  give  origin  to  the  first  and  second  lumbar  nerves. 

4.  Abdominal  reflex,  consisting  of  a  contraction  of  the  abdominal 
muscles  when  the  skin  upon  the  side  of  the  abdomen  is  gently  scratched; 
it  takes  place  through  the  spinal  segments  which  give  origin  to  the  nerves 
from  the  eighth  to  the  twelfth  thoracic. 

5.  Epigastric  reflex,  consisting  of  a  slight  musclar  contraction  in  the 
neighborhood  of  the  epigastrium  when  the  skin  between  the  fourth  and 
sixth  ribs  is  stimulated;  it  takes  place  through  the  segments  of  the  cord 
which  gives  origin  to  the  nerves  from  the  fourth  to  the  seventh  thoracic 
inclusive. 

6.  Scapular  reflex,  consisting  of  a  contraction  of  the  scapular  muscle 
when  the  skin  between  the  scapulas  is  stimulated;  it  takes  place 
through  the  segments  of  the  cord  which  gives  rise  to  the  nerves  from 
the  fifth  cervical  to  the  third  thoracic  inclusive. 

The  skin  or  superficial  reflexes,  though  variable,  are  generally  present  in 
health.  They  are  increased  or  exaggerated  when  the  gray  matter  of  the 
cord  is  abnormally  excited,  as  in  tetanus,  strychnin-poisoning,  and  disease 
of  the  lateral  columns. 

The  so-called  ^'tendon  reflexes"  are  characterized  by  a  movement  of  cer- 
tain parts  of  the  body  due  to  the  contraction  of  certain  muscles  and  are  eli- 
cited by  a  sharp  tap  on  their  tendons.  The  fundamental  condition  for  the 
production  of  the  tendon  reflex  is  a  certain  degree  of  tonus  of  the  muscle, 
which  is  a  true  reflex,  maintained  by  afferent  nerve  impulses  developed 
in  the  muscle  itself  in  consequence  of  its  extension  and  hence  compression 
of  the  end-organs,  the  muscle  spindles,  of  the  afferent  nerves.  When 
the  muscle  is  passively  extended,  as  it  must  be  when  the  reflex  is  to  be 
elicited,  there  is  an  exaltation  of  the  tonus  and  an  increase  in  the  irri- 
tability. To  this  condition  of  the  muscle  due  to  passive  tension,  the 
term  myotatic  irritability  has  been  given.  If  the  muscle  extension  be 
now  suddenly  increased,  as  it  is  when  the  tendon  is  sharply  tapped,  the 
increased  compression  of  the  muscle  spindles  will  develop  additional 
afferent  impulses  which  after  transmission  to  the  spinal  cord  will  give 


THE  FUNCTIONS  OF  THE  GRAY  MATTER        1 77 

rise  to  contraction  of  the  corresponding  muscle.     The  tendon  reflexes 
are  of  much  value  in  the  diagnosis  of  certain  lesions  of  the  spinal  cord. 
The  following  are  the  principal  forms  of  the  tendon  reflexes: 

1.  The  Patellar  tendon  reflex  or  knee-jerk.  This  phenomenon  is  charac- 
terized by  a  quick  extension  of  the  leg  from  the  knee  downward,  due  to 
the  contraction  of  the  extensor  muscles  of  the  thigh,  when  the  ligamentum 
patellae  is  struck  between  the  patella  and  tibia.  This  reflex  is  best 
observed  when  the  legs  are  freely  hanging  over  the  edge  of  a  table.  The 
patella  reflex  is  generally  present  in  health,  being  absent  in  only  2  per 
cent. ;  it  is  greatly  exaggerated  in  lateral  sclerosis,  in  descending  degenera- 
tion of  the  cord. ;  it  is  absent  in  locomotor  ataxia  and  in  atrophic  lesions 
of  the  anterior  gray  cornua. 

2.  The  tendo- Achilles  reflex  or  ankle-jerk.  This  phenomenon  is  char- 
acterized by  a  flexion  of  the  foot  due  to  a  contraction  of  the  gastrocnemius 
muscle  when  the  tendo-Achillis  is  struck.  To  elicit  the  contraction, 
the  leg  should  be  extended  and  the  dorsum  of  the  foot  be  pressed  toward 
the  leg  so  as  to  give  to  the  gastrocnemius  a  slight  degree  of  extension.  If 
the  tendon  be  now  sharply  struck  a  quick  flexion  of  the  foot  is  produced. 

3.  Ankle  clonus. — This  phenomenon  consists  of  a  series  of  rhythmic 
contractions  of  the  gastrocnemius  muscle,  varying  in  frequency  from  six 
to  ten  per  second.  To  elicit  this  reflex,  pressure  is  made  upon  the  sole 
of  the  foot  so  as  to  extend  the  foot  at  the  ankle  suddenly  and  energetically, 
thus  putting  the  tendo-Achillis  and  the  gastrocnemius  muscle  on  the 
stretch.  The  rhythmic  movements  thus  produced  continue  so  long  as 
the  tension  within  limits  is  maintained.  Ankle  clonus  is  never  present 
in  health,  but  is  very  marked  in  lateral  sclerosis  of  the  cord. 

4.  The  Toe  reflex. — This  phenomenon  is  characterized  by  a  flexion 
of  the  foot,  then  of  the  leg  and  perhaps  of  the  thigh  when  the  great  toe  is 
strongly  and  suddenly  flexed.  It  is  present  in  those  diseases  of  the  spinal 
cord  in  which  there  is  a  pronounced  patellar  reflex. 

5.  The  Wrist  and  Elbow  reflex. — These  phenomena  are  characterized  by 
an  extension  movement  of  the  hand  and  arm  when  the  tendons  of  the 
extensor  muscles  are  sharply  tapped.  These  reflexes  are  especially 
marked  in  primary  lateral  sclerosis  of  the  cord  in  the  upper  portion. 

The  organ  reflexes,  e.g.,  the  activities  of  the  geni to-urinary  organs,  the 
stomach,  intestines,  gall-bladder,  etc.,  which  are  induced  by  peripheral 
stimulation  have  been  considered  in  connection  with  the  physiologic 
action  of  these  organs.  The  genitourinary  center  is  located  in  the  lumbar 
region  of  the  spinal  cord.  In  diseased  conditions  of  this  region  the  genito- 
urinary reflexes  are  sometimes  increased,  at  other  times  decreased  or 
even  abolished. 
12 


178  HUMAN   PHYSIOLOGY 

Reflex  Irritability. — The  general  irritability  or  quickness  of  response 
of  the  mechanism  involved  in  a  reflex  act  is  approximately  represented 
by  the  length  of  time  that  elapses  between  the  application  of  a  minimal 
stimulus  and  the  appearance  of  the  muscle  response.  The  total  reflex 
time  is  a  variable  factor  in  different  individuals  and  depends  very  largely 
on  the  degree  of  irritability  of  the  intra-spinal  mechanism.  If  this  is 
increased  the  entire  duration  of  the  reflex  act  is  shorter;  if  it  is  decreased 
the  duration  is  lengthened — e.g., 

The  irritability  of  the  cord  may  be — 

1.  Increased,  by  disease  of  the  lateral  columns,  by  the  administration 
of  strychnin  and  in  frogs,  by  a  separation  of  cord  from  the  brain,  the 
latter  apparently  exerting  an  inhibitor  influence  over  the  former  and 
depressing  its  reflex  activity. 

2.  Decreased,  by  destructive  lesion  of  the  cord — e.g.,  locomotor  ataxia, 
atrophy  of  the  anterior  cornua — the  administration  of  various  drugs,  and 
in  the  frog,  by  irritation  of  certain  regions  of  the  brain.  When  the  cere- 
brum alone  is  removed  and  the  optic  lobes  are  stimulated,  the  time  elapsing 
between  the  application  of  an  irritant  to  a  sensor  surface  and  the  resulting 
movement  will  be  considerably  prolonged,  the  optic  lobes  (Setschenow's 
center)  apparently  generating  impulses  which,  descending  the  cord, 
retard  its  reflex  movement. 

Special  Nerve  Centers  in  Spinal  Cord. — Throughout  the  spinal  cord 
there  are  a  number  of  spinal  nerve  centers,  capable  of  being  excited  reflexly 
and  of  producing  complex  coordinated  movements.  Though  for  the  most 
part  independent  in  action,  they  are  subject  to  the  controlling  influences  of 
the  medulla  and  brain. 

1.  alios pinal  center,  situated  in  the  cord  between  the  lower  cervical 
and  the  third  dorsal  vertebra.  It  is  connected  with  the  dilatation  of  the 
pupil  through  fibers  which  emerge  in  this  region  and  enter  the  cervical 
sympathetic.  Stimulation  of  the  cord  in  this  locality  causes  dilatation 
of  the  pupil  on  the  same  side;  destruction  of  the  cord  is  followed  by  con- 
traction of  the  pupil. 

2.  Genitos pinal  center,  situated  in  the  lower  part  of  the  cord.  This  is  a 
complex  center,  and  comprises  a  series  of  subordinate  centers  for  the  control 
of  the  muscular  movements  involved  in  the  acts  of  defecation,  micturition, 
and  ejaculation  of  semen,  and  of  the  movements  of  the  uterus  during 
parturition,  etc. 

3.  Vaso-motor  centers,  giving  origin  to  both  vaso-constrictor  and  vaso- 
dilatator fibers,  which  are  distributed  throughout  the  cord  between  the 
first  thoracic  and  third  lumbar  nerves. 


THE  FUNCTIONS  OF  THE  GRAY  MATTER        1 79 

Though  acting  reflexly,  they  are  under  the  dominating  influence  of  the 
center  in  the  medulla. 
4.  Sweat  centers  are  also  present  in  various  parts  of  the  cord. 

Direct  or  Cerebral  Excitation. — The  activity  of  the  emissive  cells  of 
the  spinal  cord  segments,  due  to  the  arrival  of  nerve  impulses  descending 
the  spinal  cord  from  the  cerebrum,  in  consequence  of  psychic  states  of  a 
volitional  or  of  an  affective  or  emotional  character,  will  be  considered  in  a 
subsequent  paragraph  entitled  "encephalo-spinal  conduction." 

The  Structure  of  the  White  Matter. — The  white  matter  of  each  half  of 
the  spinal  cord  is  anatomically  divided  by  the  ventral  and  dorsal  roots 
into  a  ventral,  a  lateral  and  a  dorsal  funiculus  or  column.  The  white 
matter  of  each  funiculus  has  been  differentiated  into  a  number  of  special- 
ized tracts  which  have  different  origins,  destinations  and  functions.  They 
are  divided  into:  (i)  intersegmental,  (2)  ascending,  and  (3)  descending. 

I.  The  Intersegmental  Tracts. — The  intersegmental  tracts  comprise  the 
fibers  heretofore  spoken  of  as  ground  bundles,  fundamental  bundles,  etc., 
bundles  of  fibers  which  occupy  those  regions  termed  ventral,  lateral  and 
dorsal  root  zones.  In  as  much  as  they  are  limited  to  the  spinal  cord  they 
are  termed  fasciculi  proprii  of  which  there  may  be  distinguished  a  ventral, 
a  lateral  and  a  dorsal  fasciculus.  These  fibers  surround  and  for  the  most 
part  lie  close  to  the  gray  matter  throughout  its  entire  extent.  These 
fasciculi  consist  of  fibers  of  variable  length  which  have  their  origin  in  the 
intrinsic  nerve-cells  of  the  gray  matter.  From  their  origin  the  nerve 
processes  pass  outward  into  the  white  matter  on  the  same  and  the  opposite 
sides,  after  which  they  divide  into  two  branches  an  ascending  and  a  de- 
scending. After  a  variable  distance  these  branches  re-enter  the  gray 
matter  and  through  their  terminal  branches  come  into  relation  with  other 
intrinsic  nerve-cells.  By  this  means  the  segments  of  the  spinal  cord 
situated  at  different  levels  are  united  anatomically  and  associated  for  the 
performance  presumably  of  complex  reflex  activities. 

The  Ascending  Tracts  or  Fasciculi. — The  ascending  tracts  are  found 
for  the  most  part  in  the  dorsal  and  lateral  funiculi,  though  a  few  are  found 
in  the  ventral  funiculus. 

The  ascending  tracts  present  in  the  dorsal  funiculus  proceeding  from 
behind  forward  may  be  divided  into  two  main  groups,  viz.:  the  dorso- 
internal  and  the  dorso-external. 

I.  The  Dorso-internal  Tract  or  Fasciculus  ofGoll. — This  tract  lies  close  to 
the  dorso-median  fissure  and  consists  of  nerve-fibers  which  are  the  con- 
tinuations of  the  dorsal  roots  of  the  sacral,  lumbar  and  lower  thoracic 
nerves  of  the  same  side.     After  entering  the  cord  they  pass  upward  as  far 


l8o  HUMAN   PHYSIOLOGY 

as  the  lower  portion  of  the  medulla  oblongata  where  their  terminal 
branches  arborize  around  the  cells  of  a  nucleus  known  as  the  clavate  or 
gracile  nucleus. 

2.  The  Dorso-external  Tract  or  Fasciculus  of  Burdach. — This  tract  lies 
just  within  the  dorsal  horn  and  is  separated  from  the  dorso-internal  tract 
by  a  septum  of  connective  tissue  most  marked  above  the  eleventh  thoracic 
segment.  The  fibers  composing  this  tract  are  the  continuations  of  the 
dorsal  roots  of  the  upper  thoracic  and  cervical  nerves.  After  entering, 
they  cross  the  cord  obliquely  and  pass  upward  as  far  as  the  lower  border 
of  the  medulla  oblongata  where  their  terminal  branches  arborize  around 
the  cells  of  a  nucleus  known  as  the  cuneate  nucleus.  Transverse  division 
of  the  fibers  of  these  tracts  is  followed  by  a  degeneration  upward  as  far  as 
the  cuneate  and  clavate  nuclei,  indicating  that  their  trophic  center  is 
situated  at  a  lower  level. 

The  ascending  tracts  present  in  the  lateral  funiculus  are  six  in  number 
of  which  the  more  important  are  as  follows : 

1.  The  Dorsal  Spino-cerebellar  Tract,  or  Tract  of  Flechsig. — This  tract  or 
fasciculus  lies  on  the  dorsal  aspect  of  the  lateral  funiculus.  It  slightly 
increases  in  size  from  the  level  of  the  second  lumbar  nerve  up  to  the 
medulla  oblongata.  It  is  composed  of  nerve-fibers  that  have  their 
origin  in  the  nerve-cells  composing  the  vesicular  column  of  Clarke  on  the 
same  side.  From  their  origin  the  fibers  pass  obliquely  outward  to  the 
surface,  then  turn  upward  and  finally  by  way  of  the  inferior  peduncle 
enter  and  terminate  as  the  name  implies  in  the  cerebellum.  A  decussation 
of  the  fibers,  it  is  stated,  takes  place  in  the  superior  vermiform  process. 
When  transversely  divided  the  peripheral  portion  of  the  fibers  undergoes 
degeneration. 

2.  The  Lateral  Spino-cerebellar  Tract,  or  Tract  of  Gower. — This  tract  lies 
on  the  ventral  aspect  of  the  lateral  funiculus.  It  is  composed  of  fibers 
which  also  have  their  origin  in  nerve-cells  of  Clarke's  vesicular  column  on 
the  same  side  in  the  lower  portion  of  the  cord.  From  this  origin  the  fibers 
pass  outward  to  the  surface  and  then  pass  upward  as  far  as  the  pons  Varolii 
where  for  the  most  part  they  turn  backward  and  enter  the  cerebellum  by 
way  of  the  superior  peduncle.  When  transversely  divided  the  peripheral 
portion  of  the  fibers  undergoes  degeneration. 

3.  The  Lateral  Spino -thalamic  Tract. — This  tract  lies  just  internal  to  the 
lateral  spino-cerebellar  tract  and  has  frequently  been  confounded  with 
it.  It  consists  of  fibers  that  have  their  origin  in  nerve-cells  in  the  dorsal 
horns  of  the  opposite  side. 

4.  The  Ventral  Spina-thalamic  Tract. — This  tract  is  located  in  the  ventral 
fasciculus  proprius  just  in  front  of  the  ventral  horn  of  the  gray  matter. 


THE   FUNCTIONS    OF   THE    WHITE   MATTER  l8l 

The  fibers  composing  this  tract  likewise  have  their  origin  in  the  nerve- 
cells  in  the  dorsal  horn  of  the  opposite  side. 

From  their  origin  the  fibers  of  both  these  tracts  cross  to  the  opposite 
side  of  the  cord  and  pass  upward  in  the  regions  just  stated  to  terminate 
around  the  cells  of  the  lateral  and  ventral  nuclei  respectively  of  the  thala- 
mus. These  two  spino-thalamic  tracts  are  continued  by  third  neurons, 
which,  arising  in  the  cells  of  the  thalamus,  pass  upward  to  the  cells  of  the 
post-central  convolutions  of  the  cortex  of  the  cerebrum  and  are  known  as 
the  thalamo-cortical  tracts. 

The  Descending  Tracts  or  Fasciculi. — The  descending  tracts,  found  in 
the  lateral  and  ventral  funiculi,  are  four  in  number  as  follows: 

I.  The  Corticospinal  or  Pyramidal  Tract. — The  fibers  composing  this 
tract  are  located  for  the  most  part,  as  will  be  fully  stated  later,  in  the 
central  portion  of  the  cortex  of  the  cerebral  hemispheres  in  the  neighbor- 
hood of  the  central  or  Rolandic  fissure.  The  axons  of  these  cells  from 
each  hemisphere  descend  through  the  corona  radiata  to  and  through  the 
internal  capsule,  along  the  inferior  surface  of  the  crura  cerebri,  behind  the 
pons  to  the  medulla,  of  which  they  constitute  the  anterior  pyramids.  At 
this  point  the  pyramidal  tract  of  each  side  divides  into  two  portions,  viz. : 

1.  A  large  portion,  containing  from  85  to  90  per  cent,  of  the  fibers,  which 
decussates  at  the  lower  border  of  the  medulla  and  passes  downward  in 
the  posterior  part  of  the  lateral  column  of  the  opposite  side,  constituting 
the  crossed  pyramidal  tract;  as  it  descends  it  gradually  diminishes  in  size 
as  its  fibers  or  their  collaterals  enter  the  gray  matter  of  each  successive 
segment. 

2.  A  small  portion,  containing  from  15  to  10  per  cent,  of  the  fibers, 
which  does  not  decussate  at  the  medulla  but  passes  downward  on  the  inner 
side  of  the  anterior  column  of  the  same  side,  constituting  the  direct  pyra- 
midal tract  or  column  of  Tiirck.  This  tract  can  be  traced  down,  as  a  rule, 
only  as  far  as  the  mid-dorsal  region.  As  it  descends  it  becomes  smaller  as 
its  fibers  cross  the  anterior  commissure  to  enter  the  gray  matter  of  the 
opposite  side.  Thus  all  the  fibers  of  the  pyramidal  tract  from  each  cerebral 
hemisphere  eventually  are  brought  into  relation  with  the  cells  of  the  gray 
matter  of  the  opposite  side  of  the  cord. 

In  addition  to  the  tracts  mentioned  and  found  in  the  three  funiculi, 
other  tracts  are  present  but  their  anatomic  relations  and  physiologic  func- 
tions are  to  a  considerable  extent  obscure. 

THE  FUNCTIONS  OF  THE  WHITE  MATTER 

The  function  of  the  white  matter  in  general  is  the  conduction  of  nerve 
impulses  up  and  down  the  spinal  cord.     The  general  function  of  the  white 


1 82  HUMAN  PHYSIOLOGY 

matter  may  be  regarded  from  at  least  three  points  of  view,  viz.:  (i)  In- 
tersegmental conduction  mediated  by  the  fibers  composing  the  fasciculi 
proprii;  (2)  ascending  or  spino-encephalic  conduction  mediated  by  the 
spino-encephalic  tracts  or  fasciculi;  (3)  Descending  or  encephalo-spinal 
conduction  mediated  by  the  encephalo-spinal  tracts  or  fasciculi. 

Intersegmental  Conduction. — The  spinal  cord  consists  of  a  series  of 
physiologic  segments  each  of  which  has  a  special  function  and  is  associated 
through  its  related  spinal  nerve  with  a  definite  segment  of  the  body.  For 
the  harmonious  cooperation  and  coordination  of  all  the  spinal  segments  it 
is  essential  that  they  should  be  united  by  commissural  or  associative  fibers. 
This  is  accomplished  by  the  fibers  constituting  the  fasciculi  proprii.  The 
cord  thus  becomes  capable  of  complex  and  purposive  reflex  actions. 

Ascending  or  Spino-encephalic  Conduction. — The  conduction  of  nerve 
impulses  upward  necessitates  the  existence  of  special  tracts  which  have 
been  alluded  to  in  foregoing  paragraphs. 

The  exteroceptive  fibers  of  the  dorsal  roots  which  enter  the  cord  become 
related  in  part  with  nerve  cells  at  the  base  of  the  dorsal  horn.  From 
these  cells  axons  arise  which  cross  the  median  plane  and  ascend  the  cord  in 
the  lateral  and  ventral  spino-thalamic  tracts.  These  tracts  are  continued 
by  new  axons  emerging  from  the  thalamic  cells  and  ascending  to  the 
cerebral  cortex.  The  nerve  impulses,  brought  to  the  surface  by  the  ex- 
teroceptic  fibers  and  which  ascend  the  lateral  spino-thalamic  tract  evoke 
sensations  of  pain  and  temperature;  the  impulses  which  ascend  the  ventral 
spino-thalamic  tract  evoke  sensations  of  touch  and  pressure. 

The  proprioceptive  fibers  which  enter  the  cord  become  related  in  part 
with  the  cells  of  Clarke's  column  on  the  same  side.  From  the  cells 
axons  pass  outward  by  the  same  side  and  ascend  the  dorsal  and  lateral 
spinocerebellar  tracts — of  which  they  constitute  the  major  portion — as  the 
cerebellum  is  an  organ  for  the  coordination  of  muscle. 

Descending  or  Encephalo-spinal  Conduction. — The  conduction  of  nerve 
impulses  from  above  downward  necessitates  the  existence  of  tracts  of 
fibers  which  extend  from  the  cerebrum  to  various  levels  of  the  medulla 
and  spinal  cord. 

The  Corticospinal  or  Pyramidal  Tract. — ^This  tract  as  previously 
stated  associates  the  motor  area  of  the  cortex  with  efferent  nerve  cells  in 
the  ventral  gray  matter  of  the  aqueduct  of  Sylvius,  the  pons,  the  medulla 
and  spinal  cord,  from  these  cells  the  tract  is  continued  to  the  muscle  by 
the  fibers  constituting  the  motor  cranial  and  spinal  nerves  all_of  which  are 
distributed  to  skeletal  muscles.  Experimental  investigations  and  observa- 
tions of  pathologic  processes  indicate  that  the  fibers  of  this  tract  are  efferent 


THE   FUNCTIONS    OF   THE   WHITE    MATTER 


183 


pathways  for  the  transmission  of  motor  or  volitional  nerve  impulses  from 
the  cortex  to  the  spinal  segment,  a  cross  section  of  the  ventral  and  lateral 
funiculi  on  one  side  of  the  spinal  cord  in  any  part  of  its  extent  is  invariably 
followed  by  a  loss  of  activity  or  paralysis  of  the  muscles  below  the  section, 


Fig.  16. — Course  of  the  Fibers  for  Voluntary  Movement. — (Landois, 


while  electric  stimulation  of  the  peripheral  end  of  the  isolated  crossed  pyra- 
midal tract  is  followed  by  marked  characteristic  movements  of  the  muscles. 
Similar  results  follow  division  of  the  pyramidal  tract  in  any  part  of  its 
course  from  the  cerebral  cortex  downward.     Electric  stimulation  of  the 


184  HUMAN   PHYSIOLOGY 

cortical  cells  which  give  origin  to  the  pyramidal  tract  is  also  followed  by 
contraction  of  the  muscles  of  the  opposite  side,  while  their  destruction  is 
attended  by  paralysis  of  the  same  muscles.  As  the  nutrition  of  the  fibers 
is  governed  by  the  cells,  it  follows  that  when  the  axon  is  separated  from 
its  cell-body  it  degenerates.  It  has  been  found  that  a  lesion  of  the  pyra- 
midal tract  in  any  part  of  its  course  is  followed  by  descending  degeneration, 
which  is  taken  in  evidence  that  it  conducts  nerve  impulses  from  above 
downward.  Thus  experimental  investigation  and  pathologic  observation 
are  in  accord  in  the  view  that  physiologically  these  nerve-fibers  are  the 
pathways  for  the  transmission  of  motor  or  volitional  impulses  from  the 
encephalon  to  the  spinal  cord. 

THE  MEDULLA  OBLONGATA 

The  medulla  oblongata  is  the  expanded  portion  of  the  upper  part  of 
the  spinal  cord.  It  is  pyramidal  in  form  and  measures  38  mm.  in  length, 
20  mm.  in  breadth,  12  mm.  in  thickness,  and  is  divided  into  two  lateral 
halves  by  the  anterior  and  posterior  median  fissures,  which  are  continuous 
with  those  of  the  cord.  Each  half  is  again  subdivided  by  minor  grooves 
into  three  funiculi — viz.,  ventral,  lateral  and  dorsal. 

1.  The  ventral  funiculus  is  composed  partly  of  fibers  continuous  with 
those  of  the  ventral  funiculus  of  the  spinal  cord,  but  mainly  of  fibers 
derived  from  the  lateral  tract  of  the  opposite  side  by  decussation. — The 
united  fibers  can  be  traced  upward  through  the  pons  Varolii  and  crura 
cerebri,   to   the  corpus    striatum  and   cerebrum   where  they  originate. 

2.  The  lateral  funiculus  is  continuous  with  the  lateral  fasciculi  of  the 
cord;  its  fibers  in  passing  upward  take  three  directions — viz.,  an  external 
bundle  joins  the  restiform  body,  and  passes  into  the  cerebellum;  an  inter- 
nal bundle  decussates  at  the  median  line  and  joins  the  opposite  ventral 
funiculus;  a  middle  bundle  ascends  beneath  the  olivary  body,  behind  the 
pons,  to  the  cerebrum,  as  the  fasciculus  teres.  The  olivary  body  of  each 
side  is  an  oval  mass,  situated  between  the  ventral  funiculus  and  restiform 
body;  it  is  composed  of  white  matter  externally  and  gray  matter  inter- 
nally, forming  the  corpus  dentatum, 

3 .  The  dorsal  funiculus  is  a  narrow  white  cord  bordering  the  posterior 
median  fissure;  it  is  continued  upward,  in  connection  with  the  fasiculus 
teres,  to  the  cerebrum. 

The  restiform  body,  continuous  with  the  dorsal  funiculus  of  the  cord, 
also  receives  fibers  from  the  lateral  column.  As  the  restiform  bodies  pass 
upward  they  diverge  and  form  a  space  (the  fourth  ventricle),  the  floor 
of  which  is  formed  by  gray  matter,  and  then  turn  backward  and  enter  the 
cerebellum. 


THE   PONS   VAROLII  1 85 

THE  PONS  VAROLII 

The  pons  Varolii  is  united  with  the  cerebrum  above,  the  cerebellum 
behind,  and  the  medulla  oblongata  below.  It  consists  of  transverse  and 
longitudinal  fibers,  amidst  which  are  irregularly  scattered  collections  of 
gray  or  vesicular  nervous  matter. 

The  transverse  fibers  unite  the  two  lateral  halves  of  the  cerebellum. 

The  longitudinal  fibers  are  continuous — 

1.  With  the  ventral  funiculi  of  the  medulla  oblongata,  which,  interlac- 
ing with  the  deep  layers  of  the  transverse  fibers,  ascend  to  the  crura 
cerebri,    forming  their  superficial  or  fasciculated  portions. 

2.  With  fibers  derived  from  the  olivary  fasciculus,  some  of  which  pass 
to  the  tubercula  quadrigemina,  while  others,  uniting  with  fibers  from  the 
lateral  and  posterior  funiculi  of  the  medulla,  ascend  in  the  deep  or  posterior 
portions  of  the  crura  cerebri. 

The  Gray  Matter  of  the  Medulla  and  Pons  Varolii. — The  gray 
matter  of  both  the  medulla  oblongata  and  pons  is  continuous  with  that 
of  the  spinal  cord.  It  is  arranged  however  with  much  less  regularity. 
It  forms  a  thin  layer  just  beneath  the  floor  of  the  fourth  ventricle. 
Special  groups  of  nervercells  are  found  in  it,  some  of  which  give  origin 
to  different  cranial  nerves. 

Functions  of  the  Medulla  and  Pons. — By  virtue  of  the  presence  of 
nerve-cells  and  definite  tracts  of  nerve  fibers  the  structures  may  be  re- 
garded as  consisting: 

1.  Of  nerve  centers,  each  of  which  has  a  special  function,  and 

2.  Of  conducting  paths,  by  which  these  centers  are  brought  into  rela- 
tion not  only  with  one  another  but  with  the  cerebrum,  the  cerebellum 
and  the  spinal  cord. 

The  efferent  or  emissive  nerve  cells  are  excited  to  action  by  the  same 
factors  that  excite  to  action  the  motor  or  efferent  cells  of  the  spinal  cord 
(see  page  173),  viz.:  (a)  by  local  causes,  {b)  by  the  arrival  of  nerve  im- 
pulses reflected  irom  the  skin  and  (c)  by  nerve  impulses  descending  from 
the  cerebrum  or  subordinate  regions. 

The  groups  of  nerve  cells  may  be  regarded  therefore  as  centers  for  auto- 
matic, reflex  and  volitional  activities.  Some  of  these  centers  are  as 
follows: 

I.  The  cardiac  centers,  which  exert  (i)  an  accelerator  action  over  the 
heart 's  pulsations  through  nerve  fibers  emerging  from  the  spinal  cord  in 
the  roots  of  the  first  and  second  dorsal  nerves  and  reaching  the  heart 
through  the  sympathetic  nerve;  (2)  an  inhibitor  or  retarding  action  on  the 


1 86  HUMAN  PHYSIOLOGY 

rate  of  the  heart-beat  through  efferent  fibers  in  the  trunk  of  the  pneumo- 
gastric  or  vagus  nerve.     (See  pages  in  and  112.) 

2.  A  vaso-motor  center y  which  regulates  the  caliber  of  the  blood-vessels 
throughout  the  body  in  accordance  with  the  needs  of  the  organs  and  tissues 
for  blood,  through  nerve-fibers  passing  by  way  of  the  spinal  nerves  to  the 
walls  of  the  blood  vessels.     (See  page  121.) 

3.  A  respiratory  center ^  which  coordinates  the  muscles  concerned  in  the 
production  of  the  respiratory  movements.     (See  page  133.) 

4.  A  mastication  center,  which  excites  to  activity  and  coordinates  the 
muscles  of  mastication. 

5.  A  deglutition  center,  which  excites  and  coordinates  the  muscles  con- 
cerned in  the  transference  of  the  food  from  the  mouth  to  the  stomach. 

6.  An  articulation  center,  which  coordinates  the  muscles  necessary  to  the 
production  of  articulate  speech. 

7.  A  diabetic  center,  stimulation  of  which  gives  rise  to  glycosuria. 

8.  A  salivary  center,  stimulation  of  which  excites  the  discharge  of  saliva. 

The  Medulla  and  Pons  as  Conductors. — The  anterior  funiculi  of  the 
medulla  and  their  continuations  through  the  more  ventral  portions  of  the 
pons,  being  portions  of  the  general  pyramidal  tract,  serve  to  conduct 
volitional  efferent  nerve  impulses  from  higher  portions  of  the  brain  to  the 
spinal  cord.  Division  of  these  pathways  is  at  once  followed  by  a  loss  of 
volitional  control  of  the  muscles  below  the  section. 

The  dorsal  or  tegmental  portion,  containing  the  fillet,  serves  to  transmit 
afferent  nerve  impulses  from  the  spinal  cord  to  higher  portions  of  the  brain. 
Transverse  division  of  one-half  of  the  dorsal  portion  of  the  pons  is  followed 
by  complete  anesthesia  of  the  opposite  half  of  the  body  without  any 
impairment  of  motion. 

The  restiform  bodies  constitute  a  pathway  between  the  spinal  cord  and 
the  cerebellum.  The  transverse  fibers  of  the  pons  associate  opposite  but 
corresponding  portions  of  the  cerebellar  hemispheres. 


THE  CRURA  CEREBRI 

The  crura  cerebri  are  largely  composed  of  the  longitudinal  fibers  of  the 
pons  (anterior  funiculi,  fasciculi  teretes);  after  emerging  from  the  pons 
they  increase  in  size,  and  become  separated  into  two  portions  by  a  layer  of 
dark-gray  matter,  the  locus  niger. 

The  superficial  portion,  the  crusta,  composed  in  part  of  the  anterior 
pyramids,  constitutes  the  motor  tract,  which  terminates,  to  some  extent, 


THE   CORPORA  QUADRIGEMINA  1 87 

in  the  corpus  striatum ,  but  for  the  most  part,  in  the  cerebrum;  the  deep 
portion,  made  up  of  the  fasciculi  teretes  and  posterior  funiculi  and  acces- 
sory fibers  from  the  cerebellum,  constitutes  the  sensor  tract  (the  tegmen- 
tum) which  terminates  in  the  optic  thalamus  and  cerebrum. 

The  gray  matter  is  situated  beneath  the  aqueduct  of  Sylvius  and  contains 
groups  of  nerve-cells  which  give  origin  to  the  nerve-fibers  composing  the 
third  or  oculo-motor  nerve. 

Function. — The  crura  are  conductors  of  motor  and  sensor  impulses;  the 
gray  matter  assists  in  the  coordination  of  the  complicated  movements  of 
the  eyeball  and  iris,  through  the  motor  oculi  communis  nerve.  It  also 
assists  in  the  harmonization  of  the  general  muscular  movements,  as  section 
of  one  crus  gives  rise  to  peculiar  movements  of  rotation  and  somersaults 
forward  and  backward. 

THE  CORPORA.  QUADRIGEMINA 

The  corpora  quadrigemina  are  four  small  grayish  eminences  situated 
beneath  the  posterior  border  of  the  corpus  callosum  and  behind  the  third 
ventricle.  They  rest  upon  the  lamina  quadrigemina,  which  forms  the 
roof  of  the  aqueduct  of  Sylvius.  The  superior  pair  are  the  larger  and  are 
known  as  the  superior  quadrigeminal  bodies,  the  superior  colliculi  or  the 
pregemina;  the  inferior  pair  are  the  smaller  and  are  known  as  the  inferior 
quadrigeminal  bodies,  the  inferior  colliculi,  or  the  post-gemina. 

External  and  somewhat  inferior  to  the  corpora  quadrigemina  are  two 
small  collections  of  gray  matter  the  more  external  of  which  has  been 
termed  the  external  geniculate  body  or  the  pregeniculum,  the  more  internal 
of  which  has  been  termed  the  internal  geniculate  body  or  the  post-geniculum. 

Though  these  bodies  are  closely  associated  anatomically,  they  differ  in 
origin,  in  their  relations,  and  in  their  functions. 

The  corpora  quadrigemina  show  on  microscopic  examination  that  they 
are  composed  of  nerve-cells  and  nerve-fibers,  both  of  which  are  so  intri- 
cately arranged  that  it  is  difficult  to  trace  their  relation  one  to  another  and 
to  adjoining  structures.  Some  of  the  cells  of  the  superior  quadrigeminal 
body  give  origin  to  axons  which  pass  downward  and  forward  and  terminate 
in  brush-like  expansions  around  the  nuclei  of  origin  of  the  oculo-motor, 
trochlear,  and  abducent  nuclei;  other  cells  are  surrounded  by  the  terminal 
branches  of  some  of  the  fibers  of  the  optic  tract,  though  it  is  not  probable 
that  they  are  true  visual  fibers.  Still  other  cells  receive  the  terminal 
branches  of  axons  the  cells  of  origin  of  which  are  located  in  the  occipital 
cortex  of  the  cerebrum  and  which  reach  the  superior  quadrigeminal  body 
by  way  of  the  optic  radiation'  and  internal  capsule. 


1 88  HUMAN   PHYSIOLOGY 

The  cells  of  the  post-geminum  give  origin  to  axons  which  pass  upward, 
forward,  and  outward,  enter  the  internal  capsule,  and  pass  by  way  of  the 
auditory  tract  to  the  cortex  of  the  temporo-sphenoidal  region  of  the  cere- 
brum. Many  of  the  fibers  of  the  lateral  fillet,  a  portion  of  the  auditory 
tract,  terminate  in  brush-like  expansions  around  these  same  cells.  There  is 
thus  established  a  connected  pathway  between  the  cochlea  and  the  tem- 
poro-sphenoidal cortex. 

The  external  geniculate  body  is  a  terminal  station  for  a  portion  of  the 
fine  visual  fibers  coming  from  the  retina.  From  the  cells  of  this  body  new 
axons  arise  which  course  forward  and  upward,  enter  the  internal  capsule 
and  pass  by  way  of  the  optic  radiation  to  the  cortex  of  the  occipital  region 

of  the  cerebrum. 

« 

Functions. — From  the  anatomic  relation  of  the  superior  quadrigeminal 
body  (the  pre-geminum)  to  the  optic  tract,  the  inference  can  be  drawn  that 
it  is  in  some  way  essential  to  the  performance  of  various  reflex  ocular  move- 
ments and  perhaps  to  the  variations  in  size  of  the  pupil.  Experimental 
investigations  and  pathologic  changes  support  the  inference. 

Irritation  of  the  pre-geminum  in  monkeys  on  one  side  is  followed  by 
diminution  of  the  pupils  first  on  the  opposite  side  and  then  almost  immedi- 
ately on  the  same  side.  The  eyes  at  the  same  time  are  also  widely  opened 
and  the  eyeballs  turned  upward  and  to  the  opposite  side.  If  the  irritation 
be  continued,  motor  reactions  are  exhibited  in  various  parts  of  the  body. 
Destruction  of  the  pre-geminum  in  both  monkeys  and  rabbits  is  followed 
by  blindness,  dilatation  and  immobility  of  the  pupils,  with  marked  disturb- 
ance of  equilibrium  and  locomotion  (Ferrier). 

From  the  anatomic  relation  of  the  inferior  quadrigeminal  body  (the 
post-geminum)  to  the  lateral  fillet,  the  basal  tract  for  hearing,  the  inference 
may  be  drawn  that  it  is  in  some  way  connected  with  the  auditory  process. 

Stimulation  of  the  post-geminum  gives  rise  to  cries  and  various  forms  of 
vocalization.  Pathologic  states  of  this  body  are  also  attended  by  im- 
pairment of  hearing  and  disorders  of  the  equilibrium. 

From  the  foregoing  facts  it  is  probable  that  the  corpora  quadrigemina 
are  associated  with  station  and  locomotion.  Ferrier  assumes  that  in 
these  bodies  "sensory  impressions,  retinal  and  others,  are  coordinated  with 
adaptive  motor  reactions  such  as  are  involved  in  equilibration  and 
locomotion." 

CORPORA  STRIATA  AND  OPTIC  THALAMI 

The  corpora  striata  are  two  large  ovoid  collections  of  gray  matter, 
situated  at  the  base  of  the  cerebrum,  the  larger  portions  of  which  are 


CORPORA   STRIATA   AND   OPTIC   THALAMI  1 89 

embedded  in  the  white  matter,  the  smaller  portions  projecting  into  the 
anterior  part  of  the  lateral  ventricle.  Each  striated  body  is  divided  by 
a  narrow  band  of  white  matter  into  two  portions — viz.: 

1.  The  caudate  nucleus,  the  intraventricular  portion,  which  is  conic  in 
shape,  having  its  apex  directed  backward,  as  a  narrow,  tail-like  process. 

2.  The  lenticular  nucleus,  embedded  in  the  white  matter,  and  for  the 
most  part  external  to  the  ventricle.  On  the  outer  side  of  the  lenticular 
nucleus  is  found  a  narrow  band  of  white  matter,  the  external  capsule;  and 
between  it  and  the  convolutions  of  the  island  of  Reil,  a  thin  band  of 
gray  matter,  the  claustrum. 

The  corpora  striata  are  grayish  in  color,  and  when  divided,  present 
transverse  striations,from  the  intermingling  of  white  fibers  and  gray  cells. 

Functions. — The  functions  of  the  cells  composing  the  caudate  and  len- 
ticular nuclei  are  very  obscure.  It  is  stated  by  some  experimenters  that 
localized  stimulation  both  of  a  physiologic  and  pathologic  character  is 
followed  by  a  persistent  rise  of  temperature  varying  from  1°  to  2.6°C. 

The  optic  thalami  are  two  oblong  masses  situated  in  the  ventricles  pos- 
terior to  the  corpora  striata,  and  resting  upon  the  posterior  portion  of  the 
crura  cerebri.  The  internal  surface,  projecting  into  the  lateral  ventricles, 
is  white,  but  the  interior  is  grayish,  from  a  commingling  of  both  white 
fibers  and  gray  cells.  Separating  the  lenticular  nucleus  from  the  caudate 
nucleus  and  the  optic  thalamus  is  a  band  of  white  tissue,  the  internal 
capsule. 

Functions. — From  the  anatomic  relation  of  the  optic  thalami  to  the 
general  and  special  sense  nerve-tracts,  on  the  one  hand,  and  to  the  cerebral 
cortex,  on  the  other  hand,  it  is  assumed  that  they  are  connected  with  the 
production  of  sensations  both  general  and  special,  and  act  as  intermediaries 
between  the  peripheral  sense-organs  and  the  cortex. 

It  is  probable  that  in  the  thalamus  visual,  tactile,  and  labyrinthine  im- 
pressions are  received,  coordinated,  and  reflected  outward,  with  the  result 
of  producing  various  adaptive  motor  reactions  connected  with  station  and 
equilibrium.  The  thalamus  is  believed  by  some  investigators  to  act  also 
as  an  intermediary  between  emotional  states  and  their  expression  in  the 
muscles  of  the  face,  this  power  being  lost  in  certain  pathologic  conditions. 
The  power  of  regulating  the  temperature  of  the  body  has  been  also  as- 
signed to  the  thalamus,  as  destruction  of  its  anterior  extremity  is  usually 
followed  by  a  rise  in  temperature. 

The  Internal  Capsule. — The  lenticular  nucleus  is  enclosed  on  all  sides 
by  ascending  and  descending  nerve-fibers.     From  the  manner  in  which 


190 


HUMAN  PHYSIOLOGY 


they  surround  and  enclose  the  nucleus  they  have  collectively  been  called 
the  lenticular  capsule.  If  a  horizontal  section  of  the  cerebrum  be  made 
at  a  certain  level  so  as  to  cut  across  the  capsule  and  the  enclosed  nucleus 
an  appearance  similar  to  that  shown  in  Fig.  17,  will  be  presented.  That 
portion  of  the  capsule  that  lies  between  the  caudate  nucleus  and  the  optic 
thalamus  internally,  and  the  lenticular  nucleus  externally  is  known  as  the 
internal  portion  of  the  lenticular  capsule,  or  in  its  abbreviated  form  as  the 


Fig.  17. — Horizontal  Section  of  the  Internal  Capsule  Showing  the 
Position  and  Relation  of  the  Motor  Tracts  for  the  Eye,  Head,  Tongue, 
Mouth,  Shoulder  (Shi.),  Elbow  (Elb.),  Digits  of  Hand  (Dig.),  Abdomen  (Aeb.), 
Hip,  Knee  (Kn.),  Digits  of  Foot  (Dig.).  S.  Sensor  tract.  O.T.  Optic  tract. 
A.T.  Auditory  tract. 

internal  capsule,  while  that  portion  between  the  external  convex  border  of 
the  lenticular  nucleus  and  the  claustrum  is  known  as  the  external  portion 
of  the  lenticular  capsule  or  in  its  abbreviated  form  as  the  external  capsule. 
At  a  given  level  the  internal  capsule  may  be  said  to  consist  of  two  seg- 
ments or  limbs,  an  anterior,  situated  between  the  caudate  nucleus  and 
the  anterior  extremity  of  the  lenticular  nucleus,  and  a  posterior,  situated 
between  the  optic  thalamus  and  the  posterior  extremity  of  the  lenticular 
nucleus.    The  two  segments  unite  at  an  obtuse  angle,  termed  the  knee, 


CEREBRUM  I91 

which  is  directed  toward  the  median  line.     The  appearance  which  is 
presented  at  different  levels  varies  however  considerably. 

Functions. — The  internal  capsule  has  been  shown  by  the  results  both 
of  experiment  and  of  pathologic  processes  to  be,  first,  a  pathway  for  the 
transmission  of  nerve  impulses  from  the  cerebral  cortex  to  the  pons, 
medulla,  and  spinal  cord,  which  give  rise  to  contraction  of  the  muscles 
of  the  opposite  side  of  the  body;  and,  second,  a  pathway  for  the  transmis- 
sion of  nerve  impulses  coming  from  skin,  mucous  membrane,  muscles,  and 
special  sense  organs  to  the  cortex,  where  they  give  rise  to  sensations  general 
and  special.  It  is,  therefore,  the  common  motor  and  sensor  pathway. 
For  the  reason  that  it  transmits  both  motor  and  sensor  impulses,  and  for 
the  further  reason  that  it  is  frequently  the  seat  of  pathologic  lesions  which 
are  followed  by  either  a  loss  of  motion  or  sensation  or  both,  the  internal 
capsule  is  one  of  the  most  interesting  parts  of  the  central  nerve  system. 
The  motor  tract  is  confined  to  the  posterior  one-third  of  the  anterior 
segment  and  the  anterior  two-thirds  of  the  posterior  segment.  The 
sensor  tract  is  confined  to  the  posterior  one-third  of  the  posterior  segment, 
the  extreme  end  of  which  also  contains  the  optic  and  auditory  tracts. 

The  region  of  the  anterior  segment  in  front  of  the  motor  tract  con- 
tains the  fibers  of  the  fronto-cerebellar  tract,  the  function  of  which  is 
unknown. 

The  motor  region  contains  fibers  which  descend  from  the  cerebral 
cortex  to  nerve  centers  situated  in  the  gray  matter  beneath  the  aqueduct 
of  Sylvius,  in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle, 
and  in  the  anterior  horns  of  the  gray  matter  of  the  spinal  cord,  and 
in  turn  are  connected  by  the  cranial  and  spinal  nerves  with  the  muscles 
of  the  eye,  head,  face,  trunk,  and  limbs.  The  positions  occupied  by 
these  different  tracts  are  shown  in  Fig.  17. 

From  the  fact  that  the  internal  capsule  contains  efferent  or  motor 
tracts,  and  afferent  or  sensor  tracts,  it  is  evident  that  a  destructive  lesion 
of  the  motor  tract  would  be  followed  by  a  loss  of  motion;  and  of  the 
sensor  tract,  by  a  loss  of  sensation,  on  the  opposite  side  of  the  body. 

THE  CEREBRUM 

The  cerebrum  is  the  largest  portion  of  the  encephalon,  constituting 
about  85  per  cent,  of  its  total  weight.  In  shape  it  is  ovate,  convex  on  its 
outer  surface,  narrow  in  front  and  broad  behind.  It  is  divided  by  a  deep 
longitudinal  cleft  or  fissure  into  halves,  known  as  the  cerebral  hemispheres. 
The  hemispheres  are  completely  separated  anteriorly  and  posteriorly  by 
this  fissure,  but  in  their  middle  portions  are  united  by  a  broad  white  band 


192 


HUMAN  PHYSIOLOGY 


Sub 


Fig.  i8. — Diagram  showing  Fissures  and  Convolutions  on  the  Lateral 
Aspect  of  the  Left  Hemi-cerebrum. 
F,  Frontal.  P,  Parietal.  T,  Temporal  and  O,  Occipital  lobes.  5.  Fissures  of. 
Sylvius.  EPS.  Epi-sylvian.  PRS.  Pre-sylvian.  SBS.  Sub-sylvian  fissures.  C. 
Central  fissure  or  fissure  of  Rolando.  PRC.  Precentral  fissure.  SPFR.  Super- 
frontal  fissure.  MEFR.  Media-frontal  fissure.  SBFR.  Sub-frontal  fissure.  PCPC . 
Post-central  fissure.  PTL.  Parietal  fissure.  PAROC.  Par-occipital.  EXOCC. 
Ex-occipital  fissures.  SPTMP.  Super-temporal  fissure.  MTMP.  Medi-temporal 
fissure. 


Fig.  19. — Diagram  Showing  Fissures  and  Convolutions  on  the  Mesal  Aspect 
OF  the  Left  Hemi-cerebrum. 
c.  Upper  extremity  of  the  central  fissure.     PARC.    Para-central  fissure.    SPCL. 
Super-callosal  fissure.     CL.  Callosal  fissure.     OC.  Occioital  fissure.     CLC.  Calcarine 
fissure.     CLT.  Collateral  fissure. 


CEREBRUM  I 93 

of  nerve-fibers,  the  corpus  callosum.  Each  hemisphere  or  hemi-cerebrum 
is  convex  on  its  outer  aspect,  and  corresponds  in  a  general  way  with  each 
side  of  the  cavity  of  the  skull;  the  inner  or  mesial  surface  is  flat  and  forms 
the  lateral  boundary  of  the  longitudinal  fissure. 

The  surface  of  each  hemi-cerebrum  presents  a  series  of  alternate  inden- 
tations and  elevations,  known  respectively  as  fissures  or  sulci,  and  convo- 
lutions or  gyri.  A  knowledge  of  the  situation  and  extent  of  the  principal 
fissures  and  convolutions,  as  well  as  of  their  relation  one  to  another,  is 
essential  to  a  clear  understanding  of  many  physiologic  processes,  clinical 
phenomena,  and  surgical  procedures.  A  study  of  the  accompanying 
Figs.  1 8,  19,  diagrams,  with  the  aid  of  the  accompanying  legends,  will 
show  the  location  and  relations  of  the  more  important  of  these  fissures 
and  convolutions. 

Structure. — The  gray  matter  of  the  cerebrum,  about  3  mm.  thick,  is 
composed  of  five  layers  of  nerve-cells: 

1.  A  superficial  layer,  containing  a  few  small  multipolar  ganglion  cells. 

2.  Small  ganglion  cells,  pyramidal  in  shape. 

3.  A  layer  of  large  pyramidal  ganglion  cells  with  processes  running 
off  superiorly  and  laterally. 

4.  The  glandular  formation  containing  nerve-cells. 

5.  Spindle-shaped  and  branching  nerve-cells  of  a  moderate  size. 

The  ivhite  matter  consists  of  medullated  nerve-fibers  which  though 
intricately  arranged  may  be  divided  into  three  systems,  viz.:  the  com- 
missural, the  association  and  the  projection. 

The  commissural  fibers  unite  corresponding  areas  of  the  cortex  of  each 
side. 

The  association  fibers  unite  neighboring  as  well  as  distant  parts  of  the 
same  hemisphere,  and  may,  therefore,  be  divided  into  long  and  short 
fibers. 

The  projection  fibers  unite  certain  areas  of  the  cerebral  cortex  with  the 
basal  ganglia,  the  pons,  the  medulla  oblongata,  and  the  spinal  cord.  They 
are  divided  into:  (i)  afferent  fibers  which  have  their  origin  in  the  lower 
nerve  centers  at  different  levels  and  thence  pass  to  the  cortex;  and  (2) 
efferent  fibers  which  have  their  origin  in  the  cortex  and  thence  pass  to 
the  lower  nerve  centers,  terminating  at  different  levels. 

The  afferent  fibers,  the  so-called  sensor  tract,  which  transmit  nerve  im- 
pulses coming  from  the  general  periphery  and  sense-organs,  pass  through 
the  tegmentum  as  the  mesial  and  lateral  fillets,  and  thence  to  the  cortex 
directly  by  way  of  the  internal  capsule,  or  indirectly  through  the  interme- 
diation of  the  thalamic  and  subthalamic  nuclei.  The  distribution  of 
13 


194  HUMAN  PHYSIOLOGY 

these  fibers  to  the  various  areas  of  the  cortex  will  be  stated  in  following 
paragraphs. 

The  efferent  fibers  of  the  so-called  motor  tract,  which  transmit  motor  or 
volitional  nerve  impulses  from  the  cortex  to  the  pons,  medulla,  and  spinal 
cord,  emerge  from  the  layer  of  pyramidal  cells  of  the  gray  matter  of  the 
anterior  or  the  pre-central  convolution,  the  para-central  lobule,  and  imme- 
diately adjacent  areas.  From  this  origin  the  axons  descend  through  the 
white  matter  of  the  corona  radiata,  converging  toward  the  internal  cap- 
sule, into  and  through  which  they  pass,  occupying  the  anterior  two-thirds 
of  the  posterior  limb  or  segment.  Beyond  the  capsule  they  continue  to 
descend,  occupying  the  middle  three-fifths  of  the  pes  or  crusta  of  the  crus 
cerebri,  the  ventral  portion  of  the  pons,  and  eventually  the  anterior 
pyramid  of  the  medulla  oblongata.  At  this  point  the  tract  divides  into 
two  portions,  viz.: 

1.  A  large  portion,  containing  from  85  to  90  per  cent,  of  the  fibers, 
which  decussates  at  the  lower  border  of  the  medulla  and  passes  down 
the  lateral  funiculus  of  the  cord,  constituting  the  crossed  pyramidal 
tract. 

2.  A  small  portion,  containing  from  15  to  10  per  cent,  of  the  fibers, 
which  does  not  decussate  at  the  medulla,  but  passes  down  the  inner  side 
of  the  anterior  funiculus  of  the  same  side,  constituting  the  direct  pyramidal 
tract  or  column  of  Tlirck. 

After  passing  through  the  internal  capsule,  and  as  it  descends  through 
the  crus,  pons,  and  medulla,  the  pyramidal  tract  gives  off  a  number 
of  fibers  which  cross  the  median  line  and  arborize  around  the  nerve-cells 
of  the  gray  matter  beneath  the  aqueduct  of  Sylvius  (the  nuclei  of  origin 
of  the  third  and  fourth  cranial  nerves),  and  around  the  nerve-cells  in  the 
gray  matter  beneath  the  floor  of  the. fourth  ventricle  (the  nuclei  of  origin 
of  the  remainder  of  the  motor  cranial  nerves).  The  remaining  fibers 
go  to  form  the  crossed  and  direct  pyramidal  tracts  and  arborize  around  the 
cells  in  the  anterior  horn  of  the  gray  matter  of  the  opposite  side  of  the 
cord  at  successive  levels.  By  this  means  the  cortex  is  brought  into 
anatomic  and  physiologic  relation  with  the  general  musculature  of  the 
various  cranial  and  spinal  motor  nerves. 

Functions. — The  functions  of  the  cerebrum  comprehend,  in  man  at 
least,  all  that  pertains  to  sensation,  cognition,  feeling,  and  volition.  All 
subjective  experiences,  which  in  their  totality  constitute  mind,  are 
dependent  on  and  associated  with  the  anatomic  integrity  and  the  phys- 
iologic activity  of  the  cerebrum  and  its  related  sense-organs,  the  eye, 
ear,  nose,  tongue  and  skin. 


CEREBRUM  1 95 

From  an  examination  of  the  anatomic  development  of  the  brain  in 
different  classes  of  animals,  in  different  men  and  races  of  men,  and  from 
a  study  of  the  pathologic  lesions  and  the  results  of  experimental  lesions 
of  the  brain,  evidence  has  been  obtained  which  reveals  in  a  striking 
manner  the  intimate  connection  of  the  cerebrum  and  all  phases  of  mental 
activity. 

1.  Comparative  anatomy  shows  that  there  is  a  general  connection  be- 
tween the  size  of  the  brain,  its  texture,  the  depth  and  number  of  convolu- 
*tions,  and  the  exhibition  of  mental  power.  Throughout  the  entire  animal 
series,  the  increase  in  intelligence  goes  hand  in  hand  with  an  increase  in 
the  development  of  the  brain.  In  man  there  is  an  enormous  increase 
in  size  over  that  of  the  highest  animals,  the  anthropoids.  The  most 
cultivated  races  of  men  have  the  greatest  cranial  capacity;  that  of  the 
educated  European  being  about  ii6  cubic  inches,  that  of  the  Australian 
being  about  60  cubic  inches,  a  difference  of  56  cubic  inches.  Men  dis- 
tinguished for  great  mental  power  usually  have  large  and  well-developed 
brains;  that  of  Cuvier  weighed  64  ounces;  that  of  Abercrombie,  63  ounces; 
the  average  being  about  48  to  50  ounces.  Not  only  in  size,  but,  above 
all,  the  texture  of  the  brain  must  be  taken  into  consideration. 

2.  Pathology. — Any  severe  injury  or  disease  disorganizing  the  hemi- 
spheres is  at  once  attended  by  a  disturbance  or  an  entire  suspension  of 
mental  activ^ity.  A  blow  on  the  head,  producing  concussion,  or  undue 
pressure  from  cerebral  hemorrhages,  destroys  consciousness;  physical  and 
chemic  alterations  in  the  gray  matter  have  been  shown  to  coexist  with 
insanity,  and  with  loss  of  memory,  speech,  etc.  Congenital  defects  of 
organization  from  imperfect  development  are  usually  accompanied  by  a 
corresponding  deficiency  of  intellectual  power  and  of  the  higher  instincts. 
Under  these  circumstances  no  great  advance  in  mental  development  can 
be  possible,  and  the  intelligence  remains  of  a  low  grade.  In  congenital 
idiocy  not  only  is  the  brain  of  small  size,  but  it  is  wanting  in  proper  chemic 
composition,  phosphorus ,  a  characteristic  ingredient  of  the  nervous  tissue, 
being  largely  diminished  in  amount. 

3.  Experimentation  upon  the  lower  animals — e.g.,  the  removal  of  the 
cerebral  hemispheres,  is  attended  by  results  similar  to  those  observed  in 
disease  and  injury.  Removal  of  the  cerebrum  in  pigeons  produces  com- 
plete abolition  of  intelligence,  and  destroys  the  capability  of  performing 
spontaneous  movements.  The  pigeon  remains  in  a  condition  of  pro- 
found stupor,  which  is  not  accompanied,  however,  by  a  loss  of  sensation 
or  of  the  power  of  producing  reflex  or  instinctive  movements.  The  pigeon 
can  be  temporarily  aroused  by  pinching  the  feet,  loud  noises,  lights  placed 
before  the  eyes,  etc.,  but  soon  relapses  into  a  state  of  quietude,  being 


196  HUMAN   PHYSIOLOGY 

unable  to  remember  impressions  and  connect  them  with  any  train  of 
ideas,  the  faculties  of  memory,  reason,  and  judgment  being  completely 
abolished. 

4.  Experimental  interference  with  the  blood  supply  to  the  cerebrum  is 
followed  by  a  diminished  or  complete  cessation  of  its  activities. 

The  Localization  of  Fimctions  in  the  Cerebrum. — By  the  term  locali- 
zation of  functions  is  meant  the  assignment  of  definite  physiologic  func- 
tions to  definite  anatomic  areas  of  the  cerebral  cortex.  From  experiments 
made  on  the  brains  of  animals,  by  the  observation  and  association  of 
clinical  symptoms  with  pathologic  lesions  of  the  central  nerve  system,  and 
from  observation  of  the  developmental  stages  of  the  embryonic  brain,  it 
has  been  established  in  recent  years: 

1 .  That  the  general  and  special  sense-organs  of  the  body  are  associated 
through  afferent  nerve-tracts  with  definite  though  perhaps  not  sharply 
delimited  areas  of  the  cerebral  cortex;  and — 

2.  That  certain  areas  of  the  cortex  are  associated  through  efferent  nerve- 
tracts  with  special  groups  of  skeletal  or  voluntary  muscles. 

Experimental  excitation  of  a  cortical  area  associated  with  a  sense-organ 
is  undoubtedly  attended  by  the  production  of  a  sensation  at  least  similar 
to  that  produced  by  peripheral  excitation  of  the  sense-organ  itself;  destruc- 
tion of  the  area  is  followed  by  an  abolition  of  all  the  sensations  associated 
with  the  sense-organ.     For  these  reasons  such  areas  are  termed  sensor. 

Experimental  excitation  of  a  cortical  area  associated  with  a  group  of 
skeletal  muscles  is  attended  by  their  contraction;  destruction  of  the  area  is 
followed  by  their  relaxation  or  paralysis.  For  these  reasons  such  areas 
are  termed  motor. 

The  Sensor  Areas. — The  sensor  areas  which  should  theoretically  be 
present  in  the  cortex  are  primarily  those  which  receive  and  translate  into 
conscious  sensations  nerve  impulses,  developed  by  changes  going  on  in  the 
body  itself;  and  secondarily  those  which  receive  and  translate  into  con- 
scious sensations  the  nerve  impulses  developed  in  the  special  sense-organs 
by  the  impact  of  the  external  or  objective  world.  In  the  former  areas,  are 
received  the  nerve  impulses  that  come  from  the  mucous  membranes, 
muscles,  joints,  viscera,  etc.,  and  give  rise  to  muscle,  and  visceral  sensa- 
tions. In  the  latter  areas  are  received  the  nerve  impulses  that  come  from 
the  sense-organs  and  give  rise  to  cutaneous,  gustatory,  olfactory,  auditory, 
and  visual  sensations.  A  number  of  such  sense  areas  may  be  predicated: 
e.g.,  areas  of  cutaneous  and  muscle  sensibility,  of  gustatory,  olfactory,  and 
visual  sensibility. 


CEREBRUM 


197 


The  sensor  areas  occupy  regions  more  or  less  widely  separated,  though 
they  are  associated  one  with  the  other  by  association  libers  (Figs,  20,  21). 

I.  The  Cutaneous  Area. — The  area  of  cutaneous  or  tactile  sensibility 
has  been  assigned  to  the  po^-central  convolution  on  the  lateral  aspect, 
and  to  a  portion  of  the  super-frontal  convolution  and  the  lower  half 
of  the  para-central  lobule  on  the  mesial  aspect  of  the  hemicerebrum. 

Destruction  of  the  post-central  convolution  in  monkeys  by  the  electro- 
cautery and  in  man  by  disease  has  invariably  led  to  a  loss  of  sensibility, 
hemianesthesia,  on  the  opposite  side  of  the  body  without  at  the  same  time 


CONCRtTE   CONCEPT 


Fig.  20. — The  Areas  and   Centers  of  the  Lateral  Aspect  of  the   Human 
Hemicerebrum. — (C.  K.  Mills.) 

causing  any  loss  of  motion.  The  location  and  extent  of  the  anesthesia 
corresponds,  of  course,  with  the  location  and  extent  of  the  lesion  of  the 
cortex. 

2.  The  Muscle  Sense  Area. — The  area  of  muscle  sensibility  has  been 
assigned  to  the  region  posterior  to  but  adjoining  the  post-central  con- 
volution and  includes  the  anterior  part  of  the  super-parietal  and  sub- 
parietal  convolution  and  perhaps  the  supra-marginal  convolution  on  the 
mesial  aspect  of  the  hemicerebrum. 

The  sensations  which  are  evoked  in  response  to  the  action  of  nerve 


198 


HUMAN  PHYSIOLOGY 


impulses  coming  from  tendons,  muscles,  etc.,  are  those  of  passive  position 
and  the  direction  and  duration  of  movements  of  parts  of  the  body.  Clinic 
observations  and  post-mortem  findings  indicate  that  lesions  of  this  area 
are  followed  by  a  loss  of  the  muscle  sense.    ' 

In  addition  to  sensations  of  passive  position  and  direction  of  movements, 
the  sensations  of  temperature  and  deep  pressure  are  also  associated  with 
the  physiologic  activities  of  this  region  of  the  parietal  lobe. 

3.  The  Stereo  gnostic  Area. — The  area  of  stereognostic  perception. 
Stereognostic  is  the  recognition  of  any  object  when  placed  in  the  hands, 


Fig.  21.- 


-The  Areas  and   Centers  of  the  Mesial  Aspect  of  the  Human 
^Hemicerebrum. — (C  K.  Mills.) 


through  its  form,  density,  temperature,  etc.  The  area  associated  with 
stereognostic  perception  has  been  assigned  to  a  portion  of  the  super- 
parietal  convolution  and  to  the  precuneus. 

A  lesion  of  this  area  impairs  or  destroys  the  power  of  recognition  of 
objects  and  establishes  the  condition  of  aster eo gnosis. 

4.  The  Gustatory  Area. — The  area  for  gustatory  sensibility  has  been 
assigned  to  the  sub-collateral  convolution  on  the  mesial  aspect  of  the 
temporo-sphenoidal  lobe. 

Disease  processes  involving  this  area  give  rise  frequently  to  subjective 


CEREBRUM  I 99 

sensations  of  taste.     Electric  stimulation  of  this  area  in  mammals  causes 
*movements  of  the  lips,  tongue,  etc,  which  are  usually  associated  with 
sensations  of  taste. 

5.  The  Olfactory  Area. — The  area  for  olfactory  sensibility  has  been 
assigned  to  the  anterior  portion  of  the  hippocampal  convolution  (the 
uncinate  region)  and  the  anterior  portion  of  the  callosal  convolution  or 
gyrus  fornicatus.  Disease  processes  in  this  region  give  rise  frequently 
to  subjective  sensations  of  odors  which  as  a  rule  are  of  an  unpleasant  char- 
acter.    Destruction  of  this  area  is  followed  by  a  loss  of  odor  sensations. 

6.  The  Auditory  Area. — The  area  of  auditory  sensibility  has  been 
assigned  to  portions  of  the  temporal  lobe  and  many  be  divided  into 
primary  and  secondary  areas. 

The  primary  area  is  located  in  the  posterior  portion  of  the  super-tem- 
poral convolution,  and  perhaps  the  posterior  portion  of  the  insula. 

The  secondary  areas  are  located  one  below  and  in  advance  and  the  other 
below  and  somewhat  behind  the  primary  area,  both  extending  into  the 
medi-temporal  convolution. 

Unilateral  destruction  of  the  primary  area  is  followed,  however,  only 
by  a  partial  loss  of  hearing  in  the  opposite  ear,  owing  to  partial  decussation 
of  the  auditory  nerve,  which,  however,  may  be  recovered  from,  after  a 
time,  owing  probably  to  a  compensatory  activity  of  the  insular  convolu- 
tion. Bilateral  destruction  of  this  region  is  followed  by  complete  deafness. 
The  primary  area  is  connected  on  the  one  hand  with  the  basal  auditory 
center  (the  internal  geniculate  body)  by  the  auditory  radiation  and  on 
the  other  hand  with  the  secondary  areas  by  association  fibers. 

In  the  first  of  these  areas  there  are  cells  in  which  the  sounds  of  objects 
are  registered  (object  hearing) ;  in  the  second  of  these  areas  there  are  cells 
in  which  the  sounds  of  words,  letters,  etc.,  are  registered  or  memorized. 
If  these  areas  are  destroyed  by  disease  the  condition  of  object-deafness 
and  word-deafness  is  established.  If  word-deafness  alone  exists,  the 
patient  though  able  to  recognize  sounds  is  unable  to  understand  spoken 
language  and  is  in  the  condition  of  a  man  who  is  hearing  a  language  of 
which  he  has  not  the  slightest  idea.  The  same  holds  true  for  the  perception 
of  sensations  of  sound  produced  by  objects. 

In  the  temporal  lobe  there  are  other  areas,  some  of  which  are  more 
or  less  associated  with  the  auditory  nerve,  such  as  intonation,  equi- 
libratory  and  orientation  areas.  (For  the  ajfferent  pathway  to  this  area, 
see  auditory  nerve.) 

7.  The  Visual  Area. — The  area  for  visual  sensibility  has  been  assigned 
to  portions  of  the  occipital  and  parietal  lobes  and  may  be  divided  into 
primary  and  secondary  areas. 


200  ^  HUMAN   PHYSIOLOGY 

The  primary  area  is  located  in  a  triangular-shaped  area  on  the  mesial 
surface  of  the  occipital  lobe,  which  includes  the  gray  matter  above  and 
below  the  calcarine  fissure  (the  cuneus  and  upper  part  of  the  lingual  lobe), 
and  to  the  gray  matter  of  the  first  occipital  convolution  on  the  lateral 
aspect  of  the  occipital  lobe.  Focal  lesions  of  this  area  on  one  side  are  fol- 
lowed by  lateral  homonymous  hemianopsia,  which,  however,  does  not 
involve,  as  a  rule,  the  fovea  or  macula.  It  is,  therefore,  the  area  of 
homonymous  half-retinal  representation.  The  location  of  the  area  of 
macular  or  central  vision  is  near  the  anterior  extremity  of  the  calcarine 
fissure. 

The  secondary  areas  are  located  partly  on  the  lateral  aspect  of  the 
occipital  lobe  and  partly  in  the  supra-marginal  and  angular  convolutions 
of  the  parietal  lobe.  The  primary  area  is  connected,  on  the  one  hand,  with 
the  basal  visual  centers  (the  external  geniculate  body  and  the  thalamus) 
by  the  optic  radiation  and,  on  the  other  hand,  with  the  secondary  areas 
by  association  fibers. 

The  area  on  the  lateral  aspect  of  the  occipital  lobe  is  rather  extensive, 
reaching  down  as  far  as  the  third  and  fourth  occipital  convolutions.  Clin- 
ical evidence  indicates  that  the  cortex  of  this  entire  area  is  associated 
with  the  registration  or  memorization  of  the  visual  sensations  and  per- 
ceptions of  objects,  though  it  may  be  subdivided  into  smaller  areas  for 
the  registration  of  the  visual  sensations  of  different  groups  of  objects 
such  as  geometric  and  architectonic  forms,  of  persons,  places  and  natural 
objects.  Diseased  processes  in  this  region  of  the  brain  may  result  in 
the  condition  known  as  object  blindness.  The  area  on  the  lateral  aspect 
of  the  parietal  lobe  (the  supra-marginal  and  angular  convolutions)  are 
associated  with  the  memorization  of  the  visual  sensations  and  perceptions 
of  words,  letters,  numbers,  and  perhaps  objects.  If  the  visual  word  area 
is  destroyed  by  disease,  word  blindness  is  established,  and  the  patient 
is  unable  to  understand  written  or  printed  language  because  of  his  inability 
to  revive  memory  images  of  words.  Letter  and  number  blindness  may  or 
may  not  be  present  according  to  the  extent  of  the  lesion.  (For  the 
afferent  pathway  to  these  areas,  see  optic  nerve.) 

The  Motor  Areas. — The  motor  areas  which  should  theoretically  be 
present  in  the  cortex  are  those  which  in  consequence  of  the  discharge 
of  nerve  impulses  excite  contraction  of  special  groups  of  muscles  and  which 
from  their  coordinate  and  purposive  character,  are  conventionally  termed 
volitional.  Five  such  general  motor  areas  may  be  predicated:  e.g.,  one 
for  the  muscles  of  the  head  and  eyes,  one  for  the  muscles  of  the  face  and 
associated  organs,  and  others  for  the  muscles  of  the  arm,  leg,  and  trunk. 


CEREBRUM  201 

They  are  usually  designated  as  head  and  eye,  face,  arm,  leg,  and  trunk 
motor  areas. 

The  existence  and  anatomic  location  of  these  areas  in  the  cortex  of 
animals  have  been  determined  by  the  employment  of  two  methods  of 
experimentation:  viz.,  stimulation  and  destruction  or  extirpation;  the 
first  by  means  of  the  rapidly  repeated  induced  electric  currents,  the  second 
by  the  electric  cautery  and  the  knife. 

If  the  stimulation  of  a  given  area  is  attended  by  phenomena  which 
indicate  that  the  animal  is  experiencing  sensation,  and  its  destruction 
by  a  loss  of  this  capability  or  the  loss  of  a  special  sense,  it  is  assumed  that 
the  area  is  sensor  in  function — is  an  area  of  special  sense.  If  the  stimu- 
lation or  excitation  of  any  given  area  is  followed  by  contraction,  and  its 
destruction  by  paralysis  of  muscles,  it  is  assumed  that  the  area  is  motor 
in  function — is  an  area  of  motion. 

The  motor  areas  are  assigned  to  the  precentral  convolution,  the  contig- 
uous portions  of  the  base  of  the  medi-  and  subfrontal  convolutions 
and  the  paracentral  lobule. 

The  main  motor  areas  are  as  follows. 

1.  The  Head  and  Eye  Area. — This  area  has  been  assigned  to  the  con- 
tiguous portions  of  the  medi-  and  subfrontal  convolutions  just  anterior 
to  the  precentral  convolution.  It  is  subdivided  into  smaller  areas  which 
initiate  and  govern  the  movements  of  the  head  and  eyeballs.  Stimulation 
of  this  area,  in  the  chimpanzee  at  least,  produces  turning  of  the  head  to 
the  opposite  side  with  conjugate  deviation  of  the  eyes  to  that  side. 

2.  The  Face  Area. — This  area  has  been  assigned  to  the  lower  portion  of 
the  precentral  convolution  and  extends  from  below  upward  to  about  the 
level  of  the  genu  of  the  central  fissure.  This  rather  large  area  may  be 
subdivided  into  {a)  an  upper  portion  including  about  one-third  of  the 
whole  and  ijb)  a  lower  portion  including  the  remaining  two-thirds.  In 
both  the  upper  and  lower  portions,  there  are  groups  of  nerve-cells  which 
excite  to  action,  the  muscles  imparting  movements  to  (a)  the  angle  of 
the  mouth,  the  eyelids  and  jaws  and  ih)  the  movements  of  the  vocal 
bands  or  cords,  the  opening  and  closure  of  the  mouth,  the  protrusion 
and  retraction  of  the  tongue.  All  of  these  movements  have  their  areas 
of  representation  in  the  face  area. 

3.  The  Arm  Area. — This  area  has  been  assigned  to  the  precentral  con- 
volutions just  above  and  contiguous  to  the  face  area  which  it  exceeds  some- 
what in  extent.  It  is  the  largest  of  all  the  subdivisions  of  the  general  area. 
It  may  be  diviedd  into  at  least  five  smaller  areas,  the  cells  of  which  excite 
to  action  the  muscles  imparting  movements  to  the  thumb,  the  fingers,  the 
wrist,  the  elbow  and  the  shoulder. 


202  HUMAN  PHYSIOLOGY 

4.  The  Trunk  Area. — This  area  has  been  assigned  to  the  precentral 
convolution  just  superior  to  the  arm  area  and  is  rather  limited  in  extent. 
Horsely  located  a  portion  of  this  area  on  the  mesial  and  lateral  edges  of 
the  hemisphere  in  front  of  the  leg  area.  The  nerve-cells  of  this  area  when 
electrically  stimulated  excite  to  action  the  muscles,  impart  movements  to 
the  spinal  columns,  such  as  arching  rotation,  etc. 

5.  The  Leg  Area. — This  area  has  been  assigned  to  the  extreme  upper  por- 
tion of  the  precentral  convolution  and  to  the  adjoining  mesial  surface,  the 
upper  portion  of  the  paracentral  lobule.  The  area  on  the  lateral  aspect 
of  the  cerebrum  may  be  subdivided  into  at  least  four  smaller  areas  con- 
taining groups  of  nerve-cells  which  excite  to  action  the  muscles  imparting 
movements  to  the  toes,  ankle,  knee  and  hip.  Evidence  from  the  clinical 
side  has  demonstrated  the  fact  that  a  localized  irritative  lesion  of  any  one  of 
these  areas  gives  rise  to  convulsive  movements  of  the  muscles  of  the  oppo- 
site side  of  the  body,  similar  in  character  to  those  resulting  from  electric 
simulation  of  the  corresponding  areas  of  the  monkey  and  ape  brains. 
Destruction  of  these  areas  from  the  growth  of  tumors,  softening,  etc.,  is 
followed  by  paralysis  of  the  muscles.  Electric  stimulation  of  these  areas 
of  the  human  brain  for  the  purpose  of  localizing  obscure  irritative  lesions 
prior  to  surgical  procedures  on  the  brain  gives  rise  to  similar  convulsive 
movements. 

The  Motor  Speech  Area. — By  this  term  is  meant  an  area  of  the  cortex, 
the  function  of  which  is  to  arrange  language  for  outward  expression;  for 
the  use  of  the  executive  centers  concerned  with  speech,  e.g.,  the  laryngeal, 
lingual  and  facial  center  located  at  the  foot  of  the  precentral  convolution 
This  area,  i.e.,  the  motor  speech  area,  has  been  assigned  to  the  posterior 
part  of  the  subfrontal  convolution  (Broca's  convolution)  on  the  left  side  in 
those  who  are  right-handed  and  on  the  right  side  in  those  who  are  congeni- 
tally  left-handed,  and  in  the  anterior  part  of  the  insular  or  perhaps  the  pre- 
insular convolutions.  Unipolar  faradic  stimulation  of  this  area  fails  to  call 
forth  any  motor  response;  its  destruction  by  disease,  however,  is  followed 
by  a  more  or  less  extensive  loss  of  the  faculty  of  articulate  speech  or  the 
faculty  of  expressing  ideas  with  words,  a  condition  usually  spoken  of  as 
motor  aphasia  or  aphemia.  This  area  and  the  area  at  the  foot  of  the  pre- 
central convolution  are  united  by  association  fibers. 

The  Motor  Writing  Area. — By  this  term  is  meant  an  area  of  the  cortex, 
the  function  of  which  is  to  arrange  language  for  outward  projection;  for 
the  use  of  the  executive  centers  concerned  with  writing,  viz. :  the  arm  cen- 
ters located  in  the  middle  portion  of  the  precentral  convolution.  This 
area,  i.e.,  the  motor  writing  area,  has  been  assigned  to  the  posterior  half  or 
third  of  the  medi-frontal  convolution.     Unipolar  faradic  stimulation  of  this 


CEREBELLUM 


203 


area  fails  to  call  forth  any  motor  response;  its  destruction  by  disease, 
however,  is  followed  by  an  inability  to  express  ideas  by  writing,  a  condi- 
tion usually  spoken  of  as  agraphia.  This  area  and  the  general  arm  center 
in  the  precentral  convolution  are  united  by  association  fibers. 

THE  CEREBELLUM 

The  cerebellum  is  situated  in  the  inferior  fossae  of  the  occipital  bone, 
beneath  the  posterior  lobes  of  the  cerebrum.     It  attains  its  maximum 


Fig.  22. — View  of  Cerebellum  in  Section  and  of  Fourth  Ventricle  with 
THE  Neighboring  Parts. — {From  Sappey.) 
I.  Median  groove  fourth  ventricle,  ending  below  in  the  calamus  scriptorius.  with 
the  longitudinal  eminences  formed  by  the  fasciculi  teretes,  one  on  each  side  2  The 
same  groove,  at  the  place  where  the  white  streaks  of  the  auditory  nerve  emerge 
from  It  to  cross  the  floor  of  the  ventricle.  3.  Inferior  peduncle  of  the  cerebellum 
formed  by  the  restiform  body.  4.  Posterior  pyramid;  above  this  is  the  calamus 
scriptorius.  5,  S.  Superior  peduncle  of  cerebellum,  or  processus  e  cerebello  and 
testes.  6  6.  Fillet  to  the  side  of  the  crura  cerebri.  7,  7-  Lateral  grooves  of  the 
crura  cerebri.     8.  Corpora  quadrigemina.— (A/^^r  Hirschfeld  and  Leville.) 

weight,  which  is  about  one  hundred  and  forty  grams,  between  the  twenty- 
fifth  and  fortieth  years. 

It  is  composed  of  two  lateral  hemispheres  and  a  central  elongated  lobe,  the 
vermiform  process;  the  two  hemispheres  are  connected  with  each  other  by 
the  fibers  of  the  middle  peduncle,  forming  the  superficial  portion  of  the 
pons  Varolii.  The  cerebellum  is  brought  into  connection  with  the  medulla 
oblongata  and  spinal  cord  through  the  prolongation  of  the  restiform  bodies; 


204  HUMAN   PHYSIOLOGY 

with  the  cerebrum,  by  fibers  passing  upward  beneath  the  corpora  quadri- 
gemina  and  the  optic  thalami,  and  then  forming  part  of  the  diverging 
cerebral  fibers. 

Structure. — It  is  composed  of  both  white  and  gray  matter,  the  former 
being  internal,  the  latter  external,  and  is  convoluted,  for  economy  of 
space. 

The  white  matter  consists  of  a  central  stem,  the  interior  of  which  is  a 
dentated  capsule  of  gray  matter,  the  corpus  dentatum.  From  the  external 
surface  of  the  stem  of  white  matter  processes  are  given  off,  forming  the 
lamince,  see  Fig.  22,  which  are  covered  with  gray  matter. 

The  gray  matter  is  convoluted  and  covers  externally  the  laminated  proc- 
esses; a  vertical  section  through  the  gray  matter  reveals  the  following 
structures: 

1.  A  delicate  connective-tissue  layer,  just  beneath  the  pia  mater,  con- 
taining rounded  corpuscles,  and  with  branching  fibers  passing  toward  the 
external  surface. 

2.  The  cells  of  Purkinje,  forming  a  layer  of  large,  nucleated,  branched 
nerve-cells  sending  off  processes  to  the  external  layer. 

3.  A  granular  layer  of  small  but  numerous  corpuscles. 

4.  A  nerve-fiber  layer,  formed  by  a  portion  of  the  white  matter. 

Properties  and  Functions.— Irritation  of  the  cerebellum  is  not  followed 
by  any  evidences  either  of  pain  or  convulsive  movements;  it  is,  therefore, 
insensible  and  inexcitable. 

Coordination  of  Movements. — Removal  of  the  superficial  portions  of  the 
cerebellum  in  pigeons  produces  feebleness  and  want  of  harmony  in  the 
muscular  movements;  as  successive  slices  are  removed,  the  movements 
become  more  irregular,  and  the  pigeon  becomes  restless;  when  the  last  por- 
tions are  removed,  all  power  oi  flying,  walking,  standing,  etc.,  is  entirely 
gone,  and  the  equilibrium  cannot  be  maintained,  the  power  of  coordinating 
muscular  movements  being  wholly  lost.  The  same  results  have  been 
obtained  by  operating  on  all  classes  of  animals. 

The  following  symptoms  were  noticed  by  Wagner,  after  removing 
the  whole  or  a  large  part  of  the  cerebellum: 

1.  A  tendency  on  the  part  of  the  animal  to  throw  itself  on  one  side,  and 
to  extend  the  legs  as  far  as  possible. 

2.  Torsion  of  the  head  on  the  neck. 

3.  Trembling  of  the  muscles  of  the  body,  which  was  general. 

4.  Vomiting  and  occasional  liquid  evacuations. 


AUTONOMIC   NERVE    SYSTEM  205 

Forced  Movements. — Division  of  one  cms  cerebelli  causes  the  animal  to 
fall  on  one  side  and  roll  rapidly  on  its  longitudinal  axis.  According  to 
SchifT,  if  the  peduncle  be  divided  from  behind,  the  animal  falls  on  the  same 
side  as  the  injury;  if  the  section  be  made  \n  front,  the  animal  turns  to  the 
opposite  side. 

Disease  of  the  cerebellum  partially  corroborates  the  result  of  experi- 
ments; in  many  cases  symptoms  of  unsteadiness  of  gait,  from  a  want  of 
coordination,  have  been  noticed. 

Comparative  anatomy  reveals  a  remarkable  correspondence  between  the 
development  of  the  cerebellum  and  the  increase  in  complexity  of  muscular 
actions.  It  attains  a  much  greater  development,  relatively  to  the  rest  of 
the  brain,  in  those  animals  whose  movements  are  very  complex  and  varied 
in  character,  such  as  the  kangaroo,  shark,  and  swallow. 

THE  AUTONOMIC  NERVE  SYSTEM 

The  Autonomic  nerves  comprise  all  the  nerves  that  are  distributed  to 
the  non-striated  muscle-fibers  in  the  walls  of  the  blood-vessels,  in  the  walls 
of  the  viscera  and  to  the  epithelium  of  all  glands.  These  nerves  consist  of 
two  consecutively  arranged  neurons,  the  first  of  which  arises  in  the  central 
nerve  system  and  is  termed  preganglionic;  the  second  of  which  arises  in 
ganglionic  cells  and  is,  therefore,  termed  postganglionic  Inasmuch  as  the 
central  nerve-cells  giving  origin  to  the  preganglionic  fibers  are  independent 
of  volitional  control  (in  marked  contrast  to  the  nerve-cells  giving  origin  to 
the  fibers  for  skeletal  muscles)  this  system  of  nerves  possesses  a  certain 
degree  of  autonomy,  and  has  been  termed  the  autonomic  system.  Thougli 
independent  of  volitional  control  they  are  influenced  in  the  way  of 
increased  or  decreased  activity,  by  psychic  states  of  an  affective  or 
emotional  character.  Their  activity,  however,  is  mainly  excited  by  nerve 
impulses  transmitted  to  them  from  the  surfaces  of  the  body. 

The  Physiologic  Anatomy  of  the  Autonomic  Nerve  System. — In  a  con- 
sideration of  the  essential  facts  of  the  physiologic  anatomy  of  this  system 
it  will  be  convenient  to  consider  first,  the  sympathetic  ganglia  and  the 
distribution  of  their  postganglionic  fibers. 

The  Sympathetic  Ganglia.^These  ganglia  may  be  divided  into  3  groups, 
viz:  the  vertebral,  the  prevertebral  and  the  peripheral.  From  each  of  these 
groups  non-medullated  nerve-fibers  pass  in  different  directions.  The 
vertebral  ganglia  give  off  fibers  which  under  the  name  gray  rami  communi- 
cantes  pass  backward  into  the  trunks  of  the  spinal  nerves  and  are  distrib- 
uted to  the  blood-vessels  of  the  skin  of  the  trunk,  arms  and  legs,  as  well 
as  to  the  epithelium  of  the  sweat-glands  of  the  corresponding  regions. 


2o6  HUMAN  PHYSIOLOGY 

The  fibers  of  the  upper  cervical  ganglia  pass  directly  to  the  blood-vessels 
and  sweat-glands  of  the  head  and  face,  while  others  pass  directly  to 
viscera;  as  the  heart.  All  fibers  going  direct  to  their  destination  are 
termed  rami  viscerates. 

The  prevertebral  ganglia,  the  semilunar,  the  renal,  the  superior  and 
inferior  mesenteric,  give  off  fibers  which  pass  to  the  walls  of  the  blood- 
vessels and  to  the  viscera  of  the  stomach,  intestine,  gall-bladder,  liver, 
kidney  and  pelvic  viscera,  etc. 

The  peripheral  ganglia,  the  ciliary,  the  spheno-palatine,  the  otic,  the 
submaxillary,  the  cardiac,  pelvic,  etc.,  give  off  branches  which  pass  to  the 
non-striated  muscle  fibers  in  the  organs  to  which  they  are  in  anatomic 
relation. 

From  the  distribution  of  the  branches  emerging  from  all  the  different 
groups  of  ganglia,  there  is  reason  to  believe  that  they  are  directly  associ- 
ated with  vaso-aiigmentor  and  vaso-inhihitor,  viscero-augmentor  and 
viscero-inhihitor ,  secreto-motor  and  secreto -inhibitor  phenomena. 

The  Anatomic  Relation  of  the  Central  Nerve  System  to  the  Sympa- 
thetic Ganglia. — The  central  nerve  system  is  associated  anatomically  and 
physiologically  with  the  sympathetic  ganglia  through  the  intermediation 
of  fine  medullated  nerve-fibers,  the  preganglionic,  which  have  their 
origin  in  nerve-cells  situated  in  four  different  regions,  viz. : 

1.  The  Mid-brain  Region. — The  preganglionic  nerve-fibers  that  leave 
the  brain  in  this  region  arise  from  groups  of  nerve-cells  situated  high  up  in 
the  gray  matter  beneath  the  aqueduct  of  Sylvius  just  where  it  widens  to 
form  the  cavity  of  the  third  ventricle.  From  this  origin  they  enter  the 
trunk  of  the  oculo-motor  nerve  and  in  association  with  it  enter  the  orbit 
cavity.  In  this  situation  these  preganglionic  fibers  leave  the  oculo- 
motor nerve  and  enter  the  ciliary  or  ophthalmic  ganglion  around  the  nerve- 
cells  of  which  their  terminal  branches  arborize.  The  gray  postganglionic 
fibers  arising  in  the  gray  cells  of  this  ganglion  enter  the  eyeball  and  are 
ultimately  distributed  to  the  sphincter  muscle  of  the  iris  and  to  the  ciliary 
muscle. 

2.  The  Bulbar  Region. — The  preganglionic  fibers  that  leave  the  brain 
in  this  region  arise  from  nerve-cells  situated  in  the  gray  matter  beneath  the 
floor  of  the  fourth  ventricle  a  little  above  and  below  the  calamus 
scriptorius.     These  fibers  leave  this  region  by  three  routes,  viz. : 

{a.)  By  way  of  the  nerve  of  Wrisberg  or  the  pars  intermedia.  The  pre- 
ganglionic fibers  that  emerge  in  this  nerve  enter  the  facial  nerve  and 
subsequently  pass  by  way  of  the  great  superficial  petrosal  nerve  to  the 
sphenopalatine  ganglion,  and  by  the  way  of  the  chorda  tympani  nerve  to 


AUTONOMIC   NERVE   SYSTEM  207 

the  sub-maxillary  ganglion,  around  the  nerve-cells  of  which  their  terminal 
branches  arborize.  The  gray  postganglionic  fibers  which  arise  in  the  cells 
of  these  ganglia  are  distributed  to  the  blood-vessels  and  glands  of  the 
nose  and  mouth  and  to  the  blood-vessels  and  epithelium  of  the 
submaxillary  and  sublingual  glands  respectively. 

(b.)  By  way  of  the  glosso-pharyngeal  nerve.  The  fibers  that  emerge  in 
this  nerve  pass  into  the  tympanic  branch  or  nerve  of  Jacobson  and  ulti- 
mately arborize  around  the  cells  of  the  otic  ganglion.  The  gray  post- 
ganglionic fibers  which  arise  in  the  cells  of  this  ganglion  pass  by  way  of  the 
auriculo-temporal  branch  of  the  trigeminal  nerve  to  the  blood-vessels  and 
epithelium  of  the  parotid  gland. 

(c.)  By  way  of  the  vagus  nerve.  The  preganglionic  fibers  that  leave  in 
the  trunk  of  the  vagus  nerve  are  ultimately  distributed  to  the  ganglia  of 
the  heart,  stomach,  small  intestine,  etc.,  around  the  nerve-cells  of  which 
their  terminal  branches  arborize.  The  gray  postganglionic  fibers  which 
arise  in  these  ganglia  pass  to  the  heart-fibers,  to  the  non-striated  muscle- 
fibers  in  the  walls  of  the  stomach,  intestines,  etc  These  fibers  contained 
in  the  facial,  glosso-pharyngeal  and  vagus  nerves,  together  with  their 
ganglionic  continuations,  have  collectively  been  termed  the  bulbar  auto- 
nomic system.  Together  with  the  fibers  in  the  oculo-motor  nerve  they 
have  been  termed  the  cranio-bulbar  autonomic  system. 

3.  The  Mid -spinal  Cord  Region — The  preganglionic  nerve-fibers  that 
leave  the  spinal  cord  in  this  region  arise  from  groups  of  nerve-cells  situated 
in  the  gray  matter  between  the  levels  of  origin  of  the  second  thoracic 
and  the  second  and  third,  perhaps  the  fourth,  lumbar  nerves.  From  this 
origin  the  fine  pre-ganglionic  fibers  emerge  from  the  cord  in  the  ventral 
roots  of  the  thoracic  and  upper  lumbar  nerves  and  hence  naturally  fall  into 
two  groups,  viz.:  the  thoracic  and  the  lumbar.  Both  groups  of  nerves 
accompany  the  ventral  motor  roots  of  the  spinal  nerves  to  about  the  point 
where  each  nerve  divides  into  an  anterior  and  a  posterior  branch;  they  then 
leave  and  enter  the  vertebral  chain  of  ganglia.  The  branches  of  communi- 
cation are  known  as  the  white  rami  communicantes.  The  nerve-fibers 
composing  these  communicating  branches  terminate  for  the  most  part 
around  the  nerve-cells  of  the  ganglia  at  the  same  and  at  somewhat  different 
levels  and  in  different  regions. 

Some  of  the  fibers  of  the  thoracic  group,  however,  cross  the  vertebral 
chain  and  then  pass  forward  and  downward,  uniting  to  form  the  greater 
and  lesser  splanchnic  nerves,  the  terminal  branches  of  which  arborize 
around  the  cells  of  the  semilunar,  the  renal  and  the  superior  mesenteric 
ganglia.     Some  of  the  lumbar  nerves  also  pass  across  the  vertebral  chain 


208  HUMAN  PHYSIOLOGY 

to  form  the  inferior  splanchnic  nerves,  the  terminal  branches   of  which 
arborize  around  the  cells  of  the  inferior  mesenteric  ganglion. 

The  distribution  of  the  postganglionic  fibers  has  already  been  alluded 
to.  The  preganglionic  nerve-fibers  having  their  origin  in  the  mid-spinal 
cord  region  comprise  all  the  vaso-motor  (constrictor)  nerves,  the  secreto- 
motor  (sweat)  nerves  and  the  viscero-inhibitor  nerves  for  the  stomach, 
intestines  and  other  viscera,  as  well  as  some  viscero-motor  fibers.  These 
nerves  collectively  constitute  the  thoracico-lumhar  autonomic  nerve  system. 

4.  The  Sacral  Spinal-cord  Region. — The  preganglionic  nerve-fibeis 
that  leave  the  spinal  cord  in  this  region  arise  from  groups  of  nerve-cells 
situated  in  the  gray  matter  between  and  including  the  levels  of  origin  of 
the  second,  the  third  and  the  fourth  (?)  sacral  nerves.  From  this  origin 
the  preganglionic  fibers  emerge  from  the  cord  in  association  with  the  large 
motor  fibers  composing  the  ventral  roots  of  these  sacral  nerves  and  pass 
with  them  to  the  interior  of  the  pelvis.  Here  they  leave  the  sacral 
nerves  and  enter  the  pudendal  or  pelvic  nerve  (the  nervus  erigens)  and 
finally  terminate  around  the  cells  of  the  pelvic  ganglia.  From  these  gan- 
glia postganglionic  fibers  arise  which  pass  onward  to  be  distributed  to  the 
non-striated  muscle-fibers  of  pelvic  viscera  and  the  blood-vessels  of  the 
external  generative  organs.  These  fibers  contained  in  the  sacral  nerves 
together  with  their  post-ganglionic  continuation  have  collectively  been 
termed  the  sacral  autonomic  system.  It  may  be  regarded  as  a  special 
nerve  system  for  the  anal  end  of  the  gut  and  structures  developmentally 
connected  with  it. 

The  Ftmctions  of  the  Autonomic  Nerve  System. — The  functions  of 
the  autonomic  nerve  system,  as  determined  from  its  anatomic  distribu- 
tion, and  the  results  of  experimental  investigations,  are  to  augment  or  to 
inhibit  the  tonus  of  the  blood-v^essels  including  the  heart,  the  tonus  of 
visceral  walls  and  the  activity  of  the  epithelium  of  glands,  and  are,  there- ' 
fore,  the  sum  total  of  the  functions  of  the  vaso-motor,  viscero-motor  and 
secreto-motor  nerves,  that  is,  the  nerves  which  collectively  constitute 
this  system. 

In  the  various  sections  of  the  text  specific  statements  are  to  be  found  as 
to  the  functions  of  the  autonomic  nerves  in  association  with  the  oculo- 
motor nerve,  the  nerve  of  Wrisberg,  (the  great  petrosal  and  chorda 
tympani  fibers)  the  glosso-pharyngeal  nerve  (Jacobson's  nerve)  the  vagus 
nerve  (the  cardiac,  bronchial,  gastric  and  intestinal  fibers)  the  thoracico- 
lumbar  nerves  (the  vaso-motor,  viscero-motor,  secreto-motor,  and  cardiac 
accelerator  fibers)  the  sacral  nerves  (the  vaso-dilatator  and  viscero- 
motor, and  inhibitor  fibers  for  the  pelvic  viscera  and  external  organs  of 
generation. 


THE   CRANIAL   NERVES  209 

THE  CRANIAL  NERVES 

The  cranial  nerves  come  off  from  the  base  of  the  brain,  pass  through 
foramina  in  the  walls  of  the  cranium,  and  are  distributed  to  the  structures 
of  the  head,  the  face  and  in  part  to  the  organs  of  the  thorax  and  abdomen. 

According  to  the  classification  of  Soemmering,  there  are  twelve  pairs  of 
nerves,  enumerating  them  from  before  backward,  as  follows — viz.: 

First  nerve,  or  olfactory.  Seventh  nerve,  or  facial,  portio  dura. 

Second  nerve,  or  optic.  Eighth  nerve,  or  acouctic. 

Third  nerve,  or  motor  oculi  com-     Ninth  nerve,  or  glosso-pharyngeal. 

munis.  Tenth  nerve,  or  pnetimogastric. 

Fourth  nerve,  or  trochlearis.  Eleventh  nerve,  or  spinal  accessory. 

Fifth  nerve,  or  trigeminal.  Twelfth  nerve,  or  hypoglossal. 
Sixth  nerv^e,  or  abducent. 

The  cranial  nerves  may  also  be  classified  physiologically,  according  to 
their  function,  into  three  groups: 

1.  Nerves  of  special  sense — e.g.,  olfactory,  optic,  acoustic,  gustatory, 
glosso-pharyngeal  and  chorda  tympani). 

2.  Nervous  of  Motion — e.g.,  motor  oculi,  pathetic,  small  root  of  the 
trigeminal,  abducent,  facial,  spinal  accessory  and  hypoglossal. 

3.  Nerves  of  general  sensibility — e.g.,  large  root  of  the  trigeminal,  the 
glosso-pharyngeal  and  the  pneumogastric. 

ORIGINS  OF  THE  CRANIAL  NERVES 

The  nerves  of  special  sense  have  their  origin  in  neuro-epithelial  cells  in 
the  sense  organs  'with  which  they  are  connected. 

The  nerves  of  motion  have  their  origin  in  nerve-cells  situated  in  the  gray 
matter  beneath  the  floor  of  the  aqueduct  of  Sylvius  and  the  floor  of  the 
fourth  ventricle. 

The  nerves  of  general  sensibility  have  their  origin  in  the  ganglia  situated 
on  their  trunks. 

First  Nerve — Olfactory 

The  olfactory  nerve  is  situated  in  the  upper  third  of  the  nasal  fossa.  It 
consists  of  from  20  to  30  branches. 

Origin. — From  neuro-epithelial  cells  situated  among  the  epithelial  cells 
covering  the  mucous  membrane.     From  these  cells  the  nerve-fibers  pass 
upward  through  foramina  in  the  cribriform  plate  of  the  ethmoid  bone  and 
arborize  around  nerve-cells,  in  the  olfactory  bulb. 
14 


210  HUMAN  PHYSIOLOGY 

The  Olfactory  Tract. — The  olfactory  tract  consists  of  both  gray  and 
white  fibers  which  pass  from  their  origin  in  the  bulb,  to  the  base  of  the  cere- 
brum where  it  divides  three  branches,  viz.:  an  external  white  root,  which 
passes  across  the  fissure  of  Sylvius  to  the  middle  lobe  of  the  cerebrum;  an 
internal  white  root,  which  passes  also  into  the  middle  lobe;  a  gray  root,  which 
is  in  relation  with  the  anterior  lobe.  The  white  fibers  at  least  terminate 
around  nerve-cells  in  the  gray  matter  of  the  pre-callosal  part  of  the  gyrus 
fornicatus,  the  gyrus  hippocampus  and  the  gyrus  uncinatus. 

Properties. — The  olfactory  nerves  do  not  give  rise  to  either  motor  or 
sensor  phenomena  when  stimulated.  When  stimulated  at  their  periphery 
by  odorous  particles,  nerve  impulses  are  developed  which,  when  conducted 
to  the  brain,  evoke  the  sensation  of  smell.  Destruction  of  the  olfactory 
nerves,  the  bulb  or  tract,  is  followed  by  a  loss  of  the  sense  of  smell. 

Function. — Presides  over  the  sense  of  smell.  Conducts  impulses  to  the 
cerebrum  which  give  rise  to  sensations  of  odor. 

Second  Nerve — Optic 

Origin. — The  optic  nerve  arises  from  large  nerve-cells  in  the  anterior 
part  of  the  retina.  From  this  origin  the  nerve-fibers  turn  backward  and 
converge  to  form  a  well-defined  bundle  (the  optic  nerve)  which  passes  out 
of  the  eyeball,  through  the  orbit  cavity  as  far  as  the  sella  turcica.  At  this 
point  there  is  a  union  and  partial  decussation,  in  man  at  least,  of  the 
fibers,  forming  what  is  known  as  the  optic  chiasm.  From  the  posterior 
portion  of  the  chiasm  there  passes  backward  on  either  side  a  bundle  of 
nerve-fibers,  the  optic  tract.  Each  tract  contains  nerve-fibers,  which 
come  from  the  temporal  two-thirds  of  the  retina  of  the  same  side  and 
the  nasal  third  of  the  retina  of  the  opposite  side.  The  fibers  of  the 
optic  tract  arborize  around  nerve-cells  in  the  external  geniculate  body, 
the  pulvinar,  and  the  anterior  quadrigeminal  body.  By  means  of  the 
optic  radiation,  the  nerve-cells  in  these  different  ganglia  are  brought  into 
relation  with  the  visual  center,  the  cuneus. 

Properties. — The  optic  nerves  are  insensible  to  ordinary  impressions? 
and  convey  only  the  special  impressions  of  light.  Division  of  one  of  the 
nerves  is  attended  by  complete  blindness  in  the  eye  of  the  corresponding 
side. 

Hemiopia  and  Hemianopsia. — Owing  to  the  decussation  of  the  fibers  in 
the  optic  chasm,  division  of  the  optic  tract  produces  loss  of  sight  in  the 
outer  half  of  the  eye  of  the  same  side,  and  in  the  inner  half  of  the  eye  of  the 


THE   CRANIAL   NERVES  211 

opposite  side,  the  blind  part  being  separated  from  the  normal  part  by  a 
vertical  line.  The  term  hemiopia  is  applied  to  the  loss  of  function  or 
paralysis  of  the  one-half  of  the  retina;  as  a  result  of  this,  there  will  be  an 
obliteration  of  the  field  of  vision  on  th*^  opposite  side  to  which  the  term 
hemianopsia  is  given.  If,  for  example,  the  right  optic  tract  be  divided, 
there  will  be  hemiopia  in  the  outer  half  of  the  rightf  eye  and  inner  half  of 
the  left  eye,  thus  causing  left  lateral  hemianopsia,  and  as  the  two  halves 
are  affected  which  correspond  in  normal  vision,  the  condition  is  known 
as  homonymous  hemianopsia.  Lesion  of  the  anterior  part  of  the  optic 
chiasm  caused  blindness  in  the  inner  half  of  the  two  eyes. 

Functions. — Governs  the  sense  of  sight.  Receives  and  conveys  to 
the  brain  the  nerve  impulses  made  by  ether  vibrations  and  which  give 
rise  to  the  sensation  of  light. 

The  reflex  movements  of  the  iris  are  called  forth  by  stimulation  of  the 
optic  nerve.  When  light  falls  upon  the  retina,  the  nerve  impulse  devel- 
oped is  carried  back  to  the  tubercula  quadrigemina,  where  it  is  trans- 
formed into  a  motor  impulse,  which  then  passes  outward  through  the 
motor  oculi  nerve  to  the  contractile  fibers  of  the  iris  and  diminishes 
the  size  of  the  pupil.  The  absence  of  light  is  followed  by  a  dilatation  of 
the  pupil. 

Third  Nerve— The  Oculo-Motor 

Origin. — From  several  groups  of  nerve-cells  situated  in  the  gray  matter 
beneath  the  aqueduct  of  Sylvius. 

Distribution. — From  this  orig;in  the  nerve-fibers  pass  forward  and 
emerge  from  the  cerebrum  at  the  inner  side  of  the  crus  cerebri.  The  nerve 
then  passes  forward,  and  enters  the  orbit  through  the  sphenoid  fissure, 
where  it  divides  into  a  superior  branch  distributed  to  the  superior  rectus 
and  levator  palpebrce  muscles;  and  inferior  branch,  sending  branches  to  the 
internal  and  infer  or  recti  and  the  inferior  oblique  muscles;  filaments  also 
pass  into  the  cilary  or  pathalmic  ganglion;  from  this  ganglion  the  ciliary 
nerves  arise,  which  enter  the  eyeball  and  are  distributed  to  the  circular  fibers 
of  the  iris  and  the  ciliary  muscle.  This  third  nerve  also  receives  filaments 
from  the  cavernous  plexus  of  the  sympathetic  and  from  the  fifth  nerve. 

Properties. — Irritation  of  the  root  of  the  nerve  produces  contraction  of 
the  pupil,  internal  strabismus,  and  muscular  movements  of  the  eye,  but  no 
pain.  Division  of  the  nerve  is  followed  by  ptosis  (falling  of  the  upper 
eyelid) ;  external  strabismus,  due  to  the  unopposed  action  of  the  external 
rectus  muscle;  paralysis  of  the  accommodation  of  the  eye;  dilatation  of  the 


212  HUMAN   PHYSIOLOGY 

pupil  from  paralysis  of  the  circular  fibers  of  the  iris  and  ciliary  muscle; 
and  inability  to  rotate  the  eye,  slight  protrusion,  and  double  vision.  The 
images  are  crossed;  that  of  the  paralyzed  eye  is  a  little  above  that  of  the 
second,  and  its  upper  end  inclined  toward  it. 

Function. — Governs  movements  of  the  eyeball  by  innervating  all  the 
muscles  except  the  external  rectus  and  superior  oblique,  influences  the 
movements  of  the  iris,  elevates  the  upper  lid,  influences  the  accommoda- 
tion of  the  eye  for  distances.  Can  be  called  into  action  by  (i)  voluntary 
stimuli,  (2)  by  reflex  action  through  irritation  of  the  optic  nerve. 

Fourth  Nerve — Trochlearis 

Origin. — From  nerve-cells  situated  in  the  gray  matter  beneath  the 
aqueduct  of  Sylvius,  just  posterior  to  the  last  nucleus  of  the  third  nerve. 

Distribution. — The  nerve  enters  the  orbitial  cavity  through  the  sphenoid 
fissure  and  is  distributed  to  the  superior  oblique  muscle;  in  its  course  it 
receives  filaments  from  the  ophthalmic  branch  of  the  fifth  pair  and  the 
sympathetic. 

Properties. — When  the  nerve  is  irritated,  muscular  movements  are  pro- 
duced in  the  superior  oblique  muscle,  and  the  pupil  of  the  eye  is  turned 
downward  and  outward.  Division  or  paralysis  lessens  the  movements 
and  rotation  of  the  globe  downward  and  outward.  The  diplopia  conse- 
quent upon  this  paralysis  is  homonymous,  one  image  appearing  above 
the  other.  The  image  of  the  paralyzed  eye  is  below,  its  upper  end  in- 
clined toward  that  of  the  sound  eye. 

Function. — Governs  the  movements  of  the  eyeball  produced  by  the 
action  of  the  superior  oblique  muscles. 

Sixth  Nerve* — ^Abducent 

Origin. — From  nerve  -cells  situated  beneath  the  upper  half  of  the  floor 
of  the  fourth  ventricle. 

Distribution. — From  this  origin  the  nerve  passes  into  the  orbit  through 
the  sphenoid  fissure,  and  is  distributed  to  the  external  rectus  muscle. 
Receives  filaments  from  the  cervical  portion  of  the  sympathetic,  through 
the  carotid  plexus,  and  spheno-palatine  ganglion. 

*The  sixth  nerve  is  considered  in  connection  with  the  third  and  fourth  nerves 
since  they  together  constitute  the  motor  apparatus  by  which  the  ocular  muscles  are 
excited  to  action. 


THE   CRANIAL   NERVES  213 

Properties. — When  Irritated,  the  external  rectus  muscle  is  thrown  into 
conv'ulsive  movements  and  the  eyeball  is  turned  outward.  When  divided 
or  paralyzed,  this  muscle  is  paralyzed,  motion  of  the  eyeball  outward 
past  the  median  line  is  impossible,  and  the  homonymous  diplopia  increases 
as  the  object  is  moved  outward  past  this  line.  The  images  are  upon  the 
same  plane  and  parallel.  Internal  strabismus  results  because  of  the  un- 
opposed action  of  the  internal  rectus. 

Fimction. — To  innervate  the  external  rectus  muscle  by  which  the  eye- 
ball is  turned  outward. 

Fifth  Nerve — Trigeminal 

The  fifth  nerve  consists  of  both  afferent  and  efferent  fibers  which  for 
the  most  part  are  separate  and  distinct.  The  afferent  fibers  constitute  by 
far  the  major  portion,  the  efferent  fibers  the  minor  portion  of  the  nerve. 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  have  their  origin  in 
nerve-cells  in  the  Gasserian  ganglion.  From  each  cell  a  short  process 
develops  which  soon  divides  into  two  branches,  one  of  which  passes  cen- 
trally, the  other  peripherally.  The  centrally  directed  branches  form  the 
so-called  large  root;  the  peripherally  directed  branches  collectively 
constitute  the  three  main  divisions  of  the  nerve,  viz.:  the  ophthalmic, 
the  superior  maxillary  and  the  inferior  maxillary. 

Distribution. — The  centrally  directed  branches  enter  the  pons  Varolii 
on  its  lateral  aspect.  After  pursuing  a  short  distance,  these  fibers  arborize 
around  nerve-cells  in  the  gray  matter  of  the  pons  and  medulla. 

The  peripherally  directed  branches  are  distributed  as  follows: 

1.  The  ophthalmic  branches  to  the  conjunctiva  and  skin  of  the  upper 
eyelid,  the  cornea,  the  skin  of  the  forehead  and  the  nose,  the  lachrymal 
gland  and  the  mucous  membrane  of  the  nose. 

2.  The  superior  maxillary  branches  to  the  skin  and  conjunctiva  of  the 
lower  lid,  the  nose,  the  cheek  and  upper  lip,  the  palpate  teeth  of  the  upper 
jaw  and  the  alveolar  processes. 

3.  The  inferior  maxillary  branches  to  the  external  auditory  meatus,  the 
side  of  the  head,  the  mouth,  the  tongue,  the  teeth  of  the  lower  jaw,  the 
alveolar  processes  and  the  skin  of  the  lower  part  of  the  face. 

Properties. — The  trigeminal  nerve,  composed  mainly  of  afferent  fibers, 
is  the  most  acutely  sensitive  nerve  in  the  body,  and  endows  all  the  parts  to 
which  it  is  distributed  with  general  sensibility. 

Stimulation  of  the  large  root,  or  of  any  of  its  branches,  will  give  rise  to 


214  HUMAN  PHYSIOLOGY 

marked  evidence  of  pain;  the  various  forms  of  neuralgia  of  the  head  and 
face  being  occasioned  by  compression,  disease,  or  exposure  of  some  of 
its  terminal  branches. 

Division  of  the  large  root  within  the  cranium  is  followed  at  once  by  a 
complete  abolition  of  all  sensibility  in  the  head  and  face,  but  is  not  at- 
tended by  any  loss  of  motion.  The  integument,  the  mucous  membranes, 
and  the  eye  may  be  lacerated,  cut,  or  bruised,  without  the  animal  exhib- 
iting any  evidence  of  pain.  At  the  same  time  the  lachrymal  secretion  is 
diminished,  the  pupil  becomes  contracted,  the  eyeball  is  protruded,  and 
the  sensibility  of  the  tongue  is  abolished. 

The  reflex  movements  of  deglutition  are  also  somewhat  impaired,  the 
impressions  of  the  food  being  unable  to  reach  and  excite  the  nerve  center 
in  the  medulla  oblongata. 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  have  their  origin  in 
nerve-cells  in  the  gray  matter  of  the  pons  Varolii  beneath  the  floor  of  the 
fourth  ventricle. 

Distribution. — The  efferent  fibers,  known  collectively  as  the  small  root, 
emerge  from  the  side  of  the  pons  Varolii,  pass  forward  beneath  the  ganglion 
of  Gasser,  beyond  which  they  enter  the  inferior  maxillary  division.  After 
a  short  course  most  of  these  fibers  leave  the  common  trunk  and  are  dis- 
tributed to  the  muscles  of  mastication,  viz.:  the  temporal,  the  masseter, 
the  internal  and  external  pterygoid  muscles.  Other  fibers  are  distributed 
to  the  mylohyoid  muscle,  the  tensor  palati  and  the  tensor  tympani  muscles. 

Properties. — Stimulation  of  the  small  root  produces  convulsive  move- 
ments of  the  muscles  of  mastication;  section  of  the  root  causes  paralysis 
of  these  muscles,  after  which  the  jaw  is  drawn  to  the  opposite  side  by  the 
action  of  the  opposing  muscles. 

The  Influence  of  the  Trigeminal  on  the  Special  Senses. — After  division 
of  the  large  root  within  the  cranium,  a  disturbance  in  the  nutrition  of  the 
special  senses  sooner  or  later  manifests  itself. 

Sight. — In  the  course  of  twenty-four  hours  the  eye  becomes  very  vascular 
and  inflamed,  the  cornea  becomes  opaque  and  ulcerates,  the  humors  are 
discharged,  and  the  eye  is  totally  destroyed. 

Smell. — The  nasal  mucous  membrane  swells  up,  becomes  fungous,  and 
is  liable  to  bleed  on  the  slightest  irritation.  The  mucus  is  increased 
in  amount,  so  as  to  obstruct  the  nasal  passages;  the  sense  of  smell  is 
finally  abolished. 

Hearing. — At  times  the  hearing  is  impaired  from  disorders  of  nutrition 
in  the  middle  ear  and  external  auditory  meatus. 


THE   CRANIAL   NERVES  21 5 

Alteration  in  the  nutrition  of  the  special  senses  is  not  marked  if  the 
section  is  made  posterior  to  the  ganglion  of  Gasser  and  to  the  anastomos- 
ing filaments  of  the  gympathetic,  which  joins  the  nerves  at  this  point; 
but  if  the  ganglion  be  divided,  these  effects  are  very  noticeable,  owing  to 
the  section  of  the  sympathetic  filaments. 

Function. — The  trigeminal  nerve,  through  its  afferent  fibers,  endows  all 
the  parts  of  the  head  and  face  to  which  it  is  distributed  with  sensibility; 
through  its  efferent  fibers  it  gives  motion  to  the  muscles  of  mastication, 
and  to  the  tensor  muscle  of  the  palate  and  the  tensor  of  the  tympanic 
membrane;  through  anastomosing  fibers  from  the  sympathetic  it  influ- 
ences the  nutrition  of  the  special  senses. 


Seventh  Nerve — ^Facial  Nerve 

Origin. — From  a  large  nucleus  of  nerve-cells  situated  in  the  gray  matter 
beneath  the  upper  half  of  the  floor  of  the  fourth  ventricle. 

Distribution. — From  this  origin  the  nerve  emerges  from  the  lower 
border  of  the  pons.  It  then  passes  into  the  internal  auditory  meatus  in 
company  with  the  nerve  of  Wrisberg,  and  then  enters  the  aqueduct  of 
Fallopius. 

The  nerve-fibers  composing  the  nerve  of  Wrisberg  have  their  origin  in 
nerve-cells  in  the  geniculate  ganglion,  situated  on  the  facial  just  where  it 
bends  to  enter  the  aqueduct  of  Fallopius.  The  centrally  directed  branches 
enter  the  medulla  oblongata  around  the  nerve-cells  of  which  they  termin- 
ate; the  peripherally  directed  branches  enter  the  trunk  of  the  facial. 

In  the  aqueduct  the  facial  gives  off  the  following  branches — viz.: 

1.  The  large  petrosal  nerve^  which  passes  forward  to  the  s  plena  palatine, 
or  Meckel's  ganglion. 

2.  The  small  petrosal  nerve,  which  passes  to  the  otic  ganglion. 

3.  The  tympanic  branch,  which  passes  to  the  stapedius  muscle  and  en- 
dows it  with  motion. 

4.  The  chorda  tympatti  nerve,  which,  after  entering  the  posterior  part  of 
the  tympanic  cavity,  passes  forward  between  the  malleus  and  incus, 
through  the  Glasserian  fissure,  and  joins  the  lingual  branch  of  the  fifth 
nerve.  It  is  then  distributed  to  the  mucous  membrane  of  the  anterior  two- 
thirds  of  the  tongue  and  the  submaxillary  glands. 

After  emerging  from  the  stylomastoid  foramen,  the  facial  nerve  sends 
branches  to  the  muscles  of  the  ear,  the  occipitofrontalis,  the  digastric,  the 
palatoglossi,  and  palatopharyngeal;  after  which  it  passes  through  the  paro- 


2l6  HUMAN   PHYSIOLOGY 

tid  gland  and  divides  into  the  temporofacial  and  cervicofacial  branches, 
which  are  distributed  to  the  superficial  muscles  of  the  face — viz.,  occipito- 
frontalis,  corrugator  supercilii,  orbicularis  palpebrarum,  levator  labii 
superioris  et  alaeque  nasi,  buccinator,  levator  anguli  oris,  orbicularis  oris, 
zygomatic!,  depressor  anguli  oris,  platysma  myoides,  etc. 

Properties. — The  facial  is  a  motor  nerve  at  its  origin,  but  in  its  course 
receives  sensitive  filaments  from  the  fifth  pair  and  the  pneumogastric. 

Stimulation  of  the  nerve,  after  its  emergence  from  the  stylomastoid  fora- 
men, produces  convulsive  movements  in  all  the  superficial  muscles  of  the 
face.  Divisior  of  the  nerve  at  this  point  causes  paralysis  of  these  muscles 
on  the  side  of  the  section,  constituting  facial  paralysis,  the  phenomena  of 
which  are  a  relaxed  and  immobile  condition  of  the  same  side  of  the  face, 
the  eyelids  remain  open,  from  paralysis  of  the  orbicularis  palpebrarum;  the 
act  of  winking  is  abolished;  the  angle  of  the  mouth  droops,  and  saliva  con- 
stantly drains  away;  the  face  is  drawn  over  to  the  second  side;  the  face  be- 
comes distorted  upon  talking  or  laughing;  mastication  is  interfered  with, 
the  food  accumulating  between  the  gums  and  cheek,  from  paralysis  of  the 
buccinator  muscle;  fluids  escape  from  the  mouth  in  drinking;  articulation 
is  impaired,  the  labial  sounds  being  imperfectly  pronounced. 

Properties  and  Fxmctions  of  the  Branches  Given  off  in  the  Aqueduct 
of  Fallopius. 

1.  The  large  petrosal,  when  stimulated,  gives  rise  to  a  dilatation  of  the 
blood-vessels  and  a  secretion  from  the  mucous  membrane  of  nose,  soft 
palate,  upper  part  of  the  pharynx,  roof  of  the  mouth,  and  gums.  It  there- 
fore contains  vaso-motor  and  secretor  fibers, which  are  in  relation  with  the 
spheno-palatine  ganglion. 

2.  The  tympanic  branch  causes  the  stapedius  muscle  to  contract. 

3.  The  chorda  tympani  influences  the  circulation  of  the  blood  around, 
and  the  secretion  of  saliva  from,  the  submaxillary  glands,  and  through  the 
nerve  of  Wrisberg  endows  the  anterior  two- thirds  of  the  tongue  with  the 
sense  of  taste.  Stimulation  of  the  chorda  tympani  dilates  the  blood- 
vessels, increases  the  quantity  and  rapidity  of  the  stream  of  blood,  and 
increases  the  secretion  of  saliva.  Division  of  the  nerve  is  followed  by 
contraction  of  the  vessels,  and  arrest  of  the  secretion,  and  a  loss  of  the  sense 
of  taste  on  the  same  side.  It  therefore  contains  vaso-motor,  secretor  and 
gustatory  nerve-fibers. 

Function. — The  facial  is  the  nerve  of  expression,  and  coordinates  the 
muscles  employed  to  delineate  the  various  emotions,  influences  the  sense  of 
taste  and  the  secretions  of  the  submaxillary  and  sublingual  glands. 


THE   CRANIAL   NERVES  217 

Eighth  Nerve — ^Acoustic  Nerve 

The  eighth  nerve  consists  of  two  portions,  a  cochlear  or  auditory  and  a 
vestibular  or  equilihratory. 

Origin. — The  cochlear  portion  and  its  origin  in  the  bipolar  nerve-cells  of 
the  spinal  ganglion  located  in  the  spiral  canal  near  the  base  of  the  osseous 
lamina  spiralis.  The  vestibular  portion  has  its  origin  in  the  bipolar  nerve- 
cells  of  the  ganglion  of  Scarpa  located  in  the  internal  auditory  meatus. 

Distribution. — The  common  trunk  of  the  eighth  nerve,  consisting  of 
both  the  cochlear  and  vestibular  portions,  emerges  from  the  internal  audi- 
tory meatus,  after  which  it  passes  backward  and  inward  as  far  as  the  lateral 
aspect  of  the  pons,  where  the  two  main  divisions  again  separate.  The 
cochlear  portion  passes  to  the  outer  side  of  the  restiform  body;  the  vestibu- 
lar portion  passes  to  the  inner  side  of  the  restiform  body  to  the  dorsal  por- 
tion of  the  pons.  After  entering  the  pons  the  fibers  composing  both  por- 
tions come  into  histologic  relations  with  different  groups  of  nerve-cells. 

Properties. — Stimulation  of  the  cochlear  nerve  is  unattended  by  either 
motor  or  sensor  phenomena.  Division  of  the  nerve  is  followed  by  a  loss  of 
hearing.  Destruction  of  the  semicircular  canal,  involving  a  lesion  of  the 
vestibular  nerves  at  their  origin,  is  followed  by  an  impairment  of  the 
power  of  coordination  and  equilibration. 

Functions. — The  cochlear  nerve  presides  over  the  sense  of  hearing.  It 
carries  to  the  brain  the  nerve  impulses  produced  by  the  impact  of  atmos- 
pheric vibrations  on  the  ear,  and  which  give  rise  to  the  sensation  of  sound. 
The  vestibular  nerve  carries  nerve  impulses  to  the  brain,  which  excite  cer- 
tain reflex  adaptive  movements  by  which  the  equilibrium  of  the  body  is 
maintained. 

Ninth  Nerve — Glossopharyngeal 

Origin. — From  nerve-cells  in  the  ganglia  situated  on  the  trunk  of  the 
nerve  near  the  medulla  oblongata — viz.,  the  petrosal  ganglion  and  the 
jugular  ganglion.  From  these  cells  a  single  branch  emerges,  which  soon 
divides  into  two  branches,  one  of  which  passes  centrally,  the  other  pe- 
ripherally. The  centrally  directed  branches  enter  the  medulla  oblongata, 
where  they  terminate  around  nerve-cells.  The  peripherally  directed 
branches  collectively  form  the  two  main  divisions  from  which  the  nerve 
takes  its  name. 

The  glossopharyngeal  also  contains  efferent  nerve-fibers,  which  have 
their  origin  in  nerve-cells  beneath  the  floor  of  the  fourth  ventricle. 


2l8  ,  HUMAN  PHYSIOLOGY 

Distribution — The  trunk  of  the  nerve  passes  downward  and  forward, 
receiving  near  the  jugular  ganglion  fibers  from  the  facial  and  pneumogas- 
tric  nerves.  It  divides  into  two  large  branches,  one  of  which  is  distributed 
to  the  base  of  the  tongue,  the  other  to  the  pharynx.  In  its  course  it  sends 
filaments  to  the  otic  ganglion;  a  tympanic  branch  which  gives  sensibility  to 
the  mucous  membrane  of  the  fenestra  rotunda,  fenestra  ovalis,  and 
Eustachian  tube;  lingual  branches  to  the  base  of  the  tongue;  palatal 
branches  to  the  soft  palate,  uvula,  and  tonsils;  pharyngeal  branches  to 
the  mucous  membrane  of  the  pharynx. 

Properties — Irritation  of  the  roots  at  their  origin  calls  forth  evidences 
of  pain;  it  is,  therefore,  a  sensor  nerve,  but  its  sensibility  is  not  so  acute  as 
that  of  the  trigeminal.  Irritation  of  the  trunk  after  its  exit  from  the 
cranium  produces  contraction  of  the  muscles  of  the  palate  and  pharynx, 
owing  to  the  presence  of  motor  fibers. 

Division  of  the  nerve  abolishes  sensibility  in  the  structures  to  which  it  is 
distributed  and  impairs  the  sense  of  taste  in  the  posterior  third  of  the 
tongue  (see  Sense  of  Taste). 

Function — Governs  the  sensibility  of  the  pharynx,  presides  partly  over 
the  sense  of  taste,  and  controls  reflex  movements  of  deglutition  and 
vomiting. 

Tenth  Nerve — Pneumogastric.    Vagus 

Origin. — From  the  nerve-cells  situated  along  the  trunk  of  the  nerve  near 
the  medulla  oblongata — viz. :  the  jugular  and  the  plexiform  ganglia.  From 
the  nerve-cells  in  these  ganglia  a  short  process  emerges  which  soon  divides 
into  two  branches  one  of  which  passes  centrally,  the  other  peripherally. 
The  central  branches  enter  the  medulla  oblongata,  where  they  terminate 
around  nerve-cells;  the  peripheral  branches  collectively  form  the  main 
portion  of  the  trunk  of  the  nerve. 

The  pneumogastric  also  contains  efferent  fibers  which  have  their  origin 
in  nerve-cells  beneath  the  floor  of  the  medulla  oblongata.  It  also  receives 
motor  fibers  from  the  spinal  accessory,  the  facial,  the  hypoglossal  and  the 
anterior  branches  of  the  two  upper  cervical  nerves. 

Distribution. — As  the  nerve  passes  down  the  neck  it  sends  off  the  follow- 
ing main  branches: 

1.  Pharyngeal  nerves ^  which  assist  in  forming  the  pharyngeal  plexus 
which  is  distributed  to  the  mucous  membrane  and  to  the  muscles  of  the 
pharynx. 

2.  Superior  laryngeal  nerve ^   which   enters   the  larynx   through    the 


THE  CRANIAL  NERVES  219 

thyrohyoid  membrane,  and  is  distributed  to  the  mucous  membrane  lining 
the  interior  of  the  larynx,  and  to  the  cricothyroid  muscle  and  the  inferior 
constrictor  of  the  pharynx.  The  '^depressor  nerve,'"  found  in  the  rabbit, 
is  formed  by  the  union  of  two  branches,  one  from  the  superior  laryngeal, 
the  other  from  the  main  trunk;  it  passes  downward  to  be  distributed  to 
the  heart. 

3.  Inferior  laryngeal,  which  sends  its  ultimate  branches  to  all  the  in- 
trinsic muscles  of  the  larynx  except  the  cricothyroid,  and  to  the  inferior 
constrictor  of  the  pharynx. 

4.  Cardiac  branches  given  off  from  the  nerve  throughout  its  course 
which  unite  with  the  sympathetic  fibers  to  form  the  cardiac  plexus,  to 
be  distributed  to  the  heart. 

5.  Pulmonary  branches,  which  form  a  plexus  of  nerves,  and  are  dis- 
tributed to  the  bronchi  and  their  ultimate  terminations,  the  lobules  and 
air  cells. 

From  the  right  pneumogastric  nerve  branches  are  distributed  to  the 
mucous  membrane  and  the  muscular  coats  of  the  stomach  and  intestines, 
and  to  the  liver,  spleen,  kidneys,  and  suprarenal  capsules. 

Properties. — At  its  origin  the  pneumogastric  nerve  is  sensory,  as  shown 
by  direct  irritation  or  galvanization,  though  its  sensibility  is  not  very 
marked.  In  its  course  it  exhibits  motor  properties,  from  anastomosis  with 
motor  nerves. 

The  pharyngeal  branches  assist  in  giving  sensibility  to  the  mucous 
membrane  of  the  pharynx,  and  influence  reflex  phenomena  of  degluti- 
tion through  motor  fibers  which  they  contain,  derived  from  the  spinal 
accessory. 

The  superior  laryngeal  nerve  endows  the  upper  portion  of  the  larynx 
with  sensibility;  protects  it  from  the  entrance  of  foreign  bodies;  by  con- 
ducting impressions  to  the  medulla,  excites  the  reflex  movements  of 
deglutition  and  respiration;  through  the  motor  filaments  it  contains, 
produces  contraction  of  the  cricothyroid  muscle. 

Division  of  the  ^^ depressor  nerve,'^  and  stimulation  of  the  central  end 
retard  the  pulsations  of  the  heart,  and  by  depressing  the  vaso-motor 
center,  diminish  the  pressure  of  blood  in  the  large  vessels,  by  causing 
dilatation  of  the  intestinal  vessels  through  the  splanchnic  nerves. 

The  inferior  laryngeal  contains,  for  the  most  part,  motor  fibers  from  the 
spinal  accessory.  When  irritated,  produces  movement  in  the  laryngeal 
muscles.  When  divided,  is  followed  by  paralysis  of  these  muscles,  except 
the  cricothyroid,  impairment  of  phonation,  and  an  embarrassment  of  the 
respiratory  movement  of  the  larynx,  and,  finally,  death  from  suffocation. 


2  20  HUMAN   PHYSIOLOGY 

The  cardiac  branches,  through  filaments  derived  from  the  spinal  acces- 
sory, or  possibly  from  the  medulla  oblongata  direct,  exert  a  direct  inhibi- 
tory action  upon  the  heart.  Division  of  the  pneumogastrics  or  vagi  in 
the  neck  is  followed  b}^  increased  frequency  of  the  heart's  action.  Stimu- 
lation of  the  peripheral  ends  diminishes  the  heart's  pulsations,  and,  if 
sufficiently  powerful,  arrests  it  in  diastole. 

The  pulmonary  branches  give  sensibility  to  the  bronchial  mucous 
membrane  and  govern  the  movements  of  respiration.  Division  of  both 
pneumogastrics  in  the  neck  diminishes  the  frecjuency  of  the  respiratory 
movements,  which  may  fall  as  low  as  four  to  six  a  minute;  death  usually 
occurs  in  from  five  to  eight  days.  Feeble  stimulation  of  the  central  ends 
of  the  divided  nerves  accelerates  respiration,  powerful  stimulation  retards, 
and  may  even  arrest  the  respiratory  movements. 

The  gastric  branches  give  sensibility  to  the  mucous  coat,  and  through 
motor  or  efferent  fibers  give  motion  to  the  muscular  coat  of  the  stomach. 
They  influence  the  secretion  of  gastric  juice,  and  aid  the  process  of 
digestion. 

The  intestinal  branches  give  sensibility  and  motion  to  the  small 
intestines. 

Function. — A  great  sensor  nerve,  which,  through  filaments  from  motor 
sources,  influences  deglutition,  the  action  of  the  heart,  the  circulatory 
and  respiratory  systems,  voice,  the  secretions  of  the  stomach,  intestines 
and  various  glandular  organs,  and  the  contraction  of  the  walls  of  the 
stomach  and  intestines. 

Eleventh  Nerve — Spinal  Accessory 

The  spinal  accessory  nerve  consists  of  two  distinct  portions,  the  med- 
ullary or  bulbar,  and  the  spinal. 

Origin. — The  medullary  portion  has  its  origin  in  nerve-cells  in  the 
lower  part  of  the  nucleus  ambiguus,  located  beneath  the  floor  of  the  fourth 
ventricle.  From  this  origin  the  nerve-fibers  pass  forward  and  emerge 
from  the  medulla  oblongata  on  its  lateral  aspect. 

The  spinal  portion  has  its  origin  in  the  nerve-cells  located  in  the  lateral 
gray  matter  of  the  spinal  cord  as  far  down  as  the  fifth  cervical  nerve. 
From  this  origin  the  nerve-fibers  pass  to  the  surface  of  the  cord  to  emerge 
between  the  ventral  and  dorsal  roots  in  from  six  to  eight  filaments,  after 
which  they  unite  to  form  a  well-defined  nerve.  It  then  passes  into  the 
cranial  cavity  through  the  foramen  magnum  and  unites  with  the  medul- 
lary portion. 


THE    CRAKIAL    NERVES  221 

Distribution. — After  the  union  the  common  trunk  emerges  from  the 
cranial  cavity  through  the  jugular  foramen  and  after  sending  branches 
to  the  pneumogastric  and  receiving  other  in  turn  from  the  pneumogastric 
as  well  as  from  the  upper  cervical  nerves  it  divides  into  two  bran'^hes — 
viz. : 

1 .  An  internal  or  anastomotic  branch  which  soon  enters  the  trunk  of  the 
pneumogastric  nerve.  The  fibers  of  this  branch  are  ultimately  distributed 
to  some  of  the  pharyngeal  muscles;  to  all  of  the  muscles  of  the  larynx  by 
way  of  the  laryngeal  branches  of  the  vagus  nerve,  and,  according  to 
most  authorities,  to  the  heart. 

2.  An  external  branch  consisting  chiefly  of  the  accessory  fibers  from  the 
spinal  cord.  It  is  distributed  to  the  sterno-cleido-mastoid  and  trapezius 
muscles. 

Properties. — At  its  origin  it  is  a  purely  motor  nerve,  but  in  its  course 
it  exhibits  some  sensibility,  due  to  the  presence  of  anastomosing  fibers. 

Destruction  of  the  medullary  root — e.g.,  tearing  from  its  attachment  by 
means  of  forceps,  impairs  the  action  of  the  muscles  of  deglutition  and 
destroys  the  power  of  producing  vocal  sounds  from  paralysis  of  the  laryn- 
geal muscles,  without,  however,  interfering  with  the  respiratory  move- 
ments of  the  larynx,  these  being  controlled  by  other  motor  nerves.  The 
normal  rate  of  movement  of  the  heart  is  increased  by  destruction  of  the 
medullary  root. 

Irritation  of  the  external  branch  throws  the  trapezius  and  sternomastoid 
muscles  into  convulsive  movements,  though  section  of  the  nerve  does  not 
produce  complete  paralysis,  as  they  are  also  supplied  with  motor  influence 
from  the  cervical  nerves.  The  sternomastoid  and  trapezius  muscles  per- 
form movements  antagonistic,  to  those  of  respiration,  fixing  the  head, 
neck,  and  upper  part  of  the  thorax,  and  delaying  the  expiratory  movement 
during  the  acts  of  pushing,  pulling,  straining,  etc.,  and  in  the  production 
of  a  prolonged  vocal  sound,  as  in  singing.  When  the  external  branch 
alone  is  divided,  in  animals,  they  experience  shortness  in  breath  during 
exercise,  from  a  want  of  coordination  of  the  muscles  of  the  limbs  and 
respiration;  and  while  they  can  make  a  vocal  sound,  it  cannot  be 
prolonged. 

Function. — Governs  phonation  by  its  influence  upon  the  muscle 
regulating  the  position  and  tension  of  the  vocal  bands;  influences  the 
movements  of  deglutition,  inhibits  the  action  of  the  heart,  and  controls 
certain  respiratory  movements  associated  with  sustained  or  prolonged 
muscular  efforts  and  phonation. 


222  HUMAN  PHYSIOLOGY 


Twelfth  Nerve — ^Hypoglossal 

Origin. — From  nerve-cells  situated  deep  in  the  substance  of  the  medulla 
oblongata,  on  a  level  with  the  lowest  portion  of  the  floor  of  the  fourth 
ventricle.  From  this  origin  the  fibers  pass  forward  and  emerge  from  the 
medulla  in  the  groove  between  the  anterior  pyramid  and  the  olivary  body. 

Distribution. — The  trunk  formed  by  the  union  of  the  different  filaments 
passes  out  of  the  cranial  cavity  through  the  anterior  condyloid  foramen. 
After  emerging  from  the  cranium,  it  sends  filaments  to  the  sympathetic 
and  pneumogastric;  it  anastomoses  with  the  lingual  branch  of  the  fifth 
pair,  and  receives  and  sends  filaments  to  the  upper  cervical  nerves.  The 
nerve  is  finally  distributed  to  the  sternohyoid,  sternothyroid,  omohyoid, 
thyrohyoid,  styloglossi,  hyoglossi,  geniohyoid,  geniohyoglossi,  and  the 
intrinsic  muscles  of  the  tongue. 

Properties. — A  purely  motor  nerve  at  its  origin,  but  derives  sensibility 
outside  the  cranial  cavity  from  anastomosis  with  the  cervical  pneumo- 
gastric, and  fifth  nerves. 

Irritation  of  the  nerve  gives  rise  to  convulsive  movements  of  the  tongue 
and  slight  evidences  of  sensibility. 

Division  of  the  nerve  on  both  sides  abolishes  all  movements  of  the  tongue 
and  interferes  considerably  with  the  act  of  deglutition. 

When  the  hypoglossal  nerve  is  involved  in  hemiplegia,  the  tip  of  the 
tongue  is  directed  to  the  paralyzed  side  when  the  tongue  is  protruded, 
owing  to  the  unopposed  action  of  the  geniohyoglossus  on  the  sound  side. 

Articulation  is  considerably  impaired  in  paralysis  of  this  nerve,  great 
difficulty  being  experienced  in  the  pronunciation  of  the  consonantal 
sounds. 

Mastication  is  performed  with  difficulty,  from  inability  to  retain  the 
food  between  the  teeth  until  it  is  completely  triturated. 

Function. — Governs  all  the  movements  of  the  tongue  and  influences  the 
functions  of  mastication,  deglutition  and  articulation. 


THE  SENSE  OF  TOUCH 

Touch  may  be  defined  as  the  sense  by  which  pressure  or  traction  on  the 
skin  and  mucous  membrane  is  perceived. 

The  physiologic  mechanism  involved  in  the  sense  of  touch  includes  the 
skin  and  the  mucous  membrane  lining  the  mouth,  the  afferent  nerves, 


SENSE   OF   TOUCH  223 

their  cortical  connections,  and  nerve-cells  in  the  cortex  of  the  parietal 
lobe. 

Peripheral  excitation  of  this  mechanism  develops  nerve  impulses  which, 
transmitted  to  the  cortex,  evoke  sensations  of  touch  and  temperature. 
To  the  skin,  therefore,  is  ascribed  a  touch  sense  and  a  temperature  sense. 
Of  the  touch  sensations  two  kinds  may  be  distinguished:  viz.,  pressure 
sensations  and  place  sensations.  With  the  contact  of  an  external  body 
there  arises  the  perception  not  only  of  the  pressure,  but  also  the  percep- 
tion of  the  place  or  locality  of  the  contact.  In  accordance  with  this,  it  is 
customary  to  attribute  to  the  skin  a  pressure  sense  and  a  location  sense. 

The  specific  physiologic  stimuli  to  the  terminal  organs  in  the  skin  and 
oral  mucous  membrane  are  mechanic  pressure  and  thermic  vibrations. 

The  structure  of  the  skin  and  the  modes  of  termination  of  the  sensory 
nerves  have  already  been  considered  (see  page  149). 

The  touch  sense  is  coextensive  with  the  skin  and  the  mucous  membrane 
of  the  mouth.  The  touch  areas,  however,  are  not  continuous  but  discrete 
and  vary  in  number  in  each  square  centimeter  of  skin.  Thus  in  the  skin 
of  the  calf  15  touch  spots  or  areas  have  been  counted;  in  the  palm  of  the 
hand  40  to  50.  Stimulation  with  a  fine  bristle  of  such  an  area  calls  forth 
the  sensation  of  touch.  In  the  tip  of  the  index  finger  the  touch  sense  is 
quite  acute  and  associated  with  the  presence  of  touch  corpuscles  of  which 
there  are  about  20  to  each  square  millimeter  of  surface. 

The  pressure  sense  varies  with  the  sensitivity  of  the  skin,  which  varies 
in  different  parts  of  the  body  in  accordance  with  the  size  of  the  area 
pressed. 

The  place  or  location  sense  is  the  localization  of  a  sensation  to  the  place 
stimulated.  This  holds  true  not  only  for  two  or  more  points  near  or 
widely  separated  on  the  same  side,  but  also  for  corresponding  points  on 
opposite  sides  of  the  body,  even  when  the  stimuli  have  the  same  intensity 
and  are  simultaneously  applied. 

The  delicacy  of  the  localizing  power  in  any  part  of  the  skin  is  determined 
by  testing  the  power  which  the  part  possesses  of  distinguishing  the  sensa- 
tions produced  by  the  contact  of  the  points  of  a  pair  of  compasses  placed 
close  together.  The  distance  to  which  the  points  must  be  separated  in 
order  to  evoke  two  separate  recognizable  sensations  is  a  measure  of  the 
diameter  of  the  sensor  circle.  Within  this  circle  the  two  sensations  become 
fused  into  one  sensation.  The  discriminative  sensibility  of  different 
regions  as  determined  by  compass  points  is  shown  in  the  following  table; 
the  numbers  represent  the  distances  at  which  two  sensations  are 
recognized: 


2  24  HUMAN   PHYSIOLOGY 

mm. 

Tip  of  tongue i  .  i 

Palmar  surface  of  third  phalanx  of  index-finger 2.2 

Red  surface  of  lips 4.5 

Palmar  surface  of  first  phalanx  of  finger 5.5 

Tip  of  nose    6.8 

Palm  of  hand    4k 8.9 

Lower  part  of  forehead 22.6 

Dorsum  of  hand 31.6 

Dorsum  of  foot 40 . 6 

Middle  of  the  back    67.7 

The  temperature  sense  is  the  recognition  of  changes  in  the  temperature 
of  the  skin  produced  in  a  variety  of  ways  through  the  sensations  of  heat 
and  cold.  This  sense  depends  on  the  fact  that  all  over  the  skin  there  are 
small  areas  some  of  which  respond  to  warm,  others  to  cold  objects  and  are 
therefore  called  hot  and  cold  spots.  When  stimulated  they  call  forth 
sensations  of  heat  and  cold. 


THE  SENSE  OF  TASTE 

The  sense  of  taste  may  be  defined  as  the  sense  by  which  the  specific 
quality  or  flavor  of  a  substance,  applied  to  the  taste  organ,  is  perceived. 
This  sense  is  located  mainly  in  the  mucous  membrane  covering  the  surface 
of  the  tongue. 

The  physiologic  mechanism  involved  in  the  sense  of  taste  includes  the 
tongue,  the  gustatory  nerves  (contained  in  the  trunks  of  the  chorda 
tympani  and  glosso-pharyngeal  nerves)  their  cortical  connections  and 
nerve-cells  in  the  gray  matter  of  the  sub-collateral  convolution.  The 
peripheral  excitation  of  this  apparatus  gives  rise  to  nerve  impulses  which 
transmitted  to  the  brain  evoke  the  sensations  of  taste.  The  specific 
physiologic  stimulus  is  matter,  organic  and  inorganic,  in  a  state  of  solution. 

Taste  Buds  or  Beakers. — The  peripheral  ends  of  the  taste  nerves  are 
provided  with  small  ovoid  bodies  termed  taste  buds  or  beakers.  The  wall 
of  the  bud  is  composed  of  elongated  curved  epithelium  at  one  point  of 
which  there  is  a  small  opening  or  pore.  The  interior  contains  narrow 
spindle-shaped  neuro-epithelial  cells  provided  at  their  outer  extremity 
with  stiff  hair-like  filaments  which  project  into  the  taste  pore.  These 
neuro-epithelial  cells  are  in  histologic  connection  with  the  nerves  of  taste. 

The  Taste  Area. — The  taste  area,  though  confined  for  the  most  part 
to  the  tongue,  extends  in  different  individuls  to  the  mucous  membrane 
of  the  hard  palate,  to  the  anterior  surface  of  the  soft  palate,  to  the  uvula. 


SENSE    OF   SMELL  225 

the  anterior  and  posterior  half  arches,  the  tonsils,  the  posterior  wall  of 
the  pharynx,  and  the  epiglottis. 

The  Taste  Sensations. — The  sensations  which  arise  in  consequence  of 
impressions  made  by  different  substances  on  the  peripheral  apparatus  of 
this  area  are  in  so  many  instances  combinations  of  taste,  touch,  tempera- 
ture and  smell  that  they  are  extremely  difficult  of  classification.  Never- 
theless six  primary  tastes  can  be  recognized:  bitter,  sweet,  acid  or  sour, 
salt  or  saline,  alkaline  and  metallic.  Though  the  contact  of  any  bitter, 
sweet,  acid,  salt,  etc.,  substance  with  any  part  of  the  tongue  will,  if  the 
substance  be  present  in  sufficient  quantity  or  concentration,  develop  a 
corresponding  sensation,  some  regions  of  the  tongue  are  more  sensitive 
and  responsive  than  others.  Thus,  the  posterior  portion  is  more  sensitive 
to  bitter  substances  than  the  anterior;  the  reverse  is  true  for  sweet  sub- 
stances and  perhaps  for  acids  and  salines. 

The  intensity  of  the  resulting  sensation  in  any  given  instance  will  depend 
on  the  degree  of  concentration  of  the  substance,  while  its  massiveness  will 
depend  on  the  area  affected. 

The  essential  conditions  for  the  production  of  the  sensations  of  taste 
are: 

1.  A  state  of  solubility  of  the  food. 

2.  A  free  secretion  of  the  saliva,  and 

3.  Active  movements  on  the  part  of  the  tongue,  exciting  pressure 
against  the  roof  of  the  mouth,  gums,  etc.,  thus  aiding  the  solution  of 
various  articles  and  their  entrance  into  the  taste  beakers. 

THE  SENSE  OF  SMELL 

The  sense  of  smell  is  the  sense  by  which  certain  qualities  of  substances 
entering  the  nose  are  perceived. 

The  physiologic  mechanism  involved  in  the  sense  of  smell  includes  the 
nasal  fossae,  the  olfactory  nerves,  the  olfactory  tracts,  and  nerve-cells  in 
those  areas  of  the  cortex  known  as  the  uncinate  convolution  and  anterior 
part  of  the  gyrus  fornicatus.  Peripheral  stimulation  of  this  mechanism 
develops  nerve  impulses  which,  transmitted  to  the  cortex,  evoke  the  sensa- 
tions of  odor.  The  specific  physiologic  stimulus  is  matter  in  the  gaseous 
or  vaporous  state. 

For  the  appreciation  of  odorous  particles  the  air  must  be  drawn  through 

the  nasal  fossae  with  a  certain  degree  of  velocity.     If  the  particles  are 

widely  diffused  in  the  air,  they  must  be  drawn  not  only  more  quickly  but 

more  forcibly  into  contact  with  the  olfactory  hairs,  as  in  the  act  of  sniffing, 

15 


226  HUMAN   PHYSIOLOGY 

the  result  of  short  energetic  inspirations.  To  many  substances  the  ol- 
factory apparatus  is  extremely  sensitive.  Thus,  it  has  been  shown  that 
a  particle  of  mercaptan  the  actual  weight  of  which  was  calculated  to  be 
^60,000,000  of  a  milligram  gives  rise  to  a  distinct  sensation. 

The  Olfactory  Sensation. — The  sensations  which  arise  in  consequence 
of  the  excitation  of  the  olfactory  apparatus  are  very  numerous  and  their 
classification  is  extremely  difficult.  For  this  reason  it  is  customary  to 
divide  them  into  two  groups:  viz.,  agreeable  and  disagreeable,  in  accord- 
ance with  the  feelings  they  excite  in  the  individual.  As  the  olfactory 
sensations  give  rise  to  feelings  rather  than  ideas,  this  sense  plays  in  man  a 
subordinate  part  in  the  acquisition  of  knowledge.  In  lower  animals  this 
sense  is  employed  for  the  purpose  of  discovering  and  securing  food,  for 
detecting  enemies  and  friends,  and  for  sexual  purposes.  In  land  animals 
the  entire  olfactory  apparatus  is  well  developed  and  the  sense  keen;  in 
some  aquatic  animals,  as  the  dolphin,  whale,  and  seal,  the  apparatus  is 
poorly  developed  and  the  sense  dull. 

THE  SENSE  OF  SIGHT 

The  physiologic  mechanism  involved  in  the  sense  of  sight  includes  the 
eyeball,  the  optic  nerve,  the  optic  tracts,  the  thalamo-occipital  tract  or 
the  optic  radiation,  and  nerve-cells  in  the  cuneus  and  adjacent  gray  matter. 
Peripheral  stimulation  of  this  mechanism  develops  nerve  impulses  which 
transmitted  to  the  cortex  evoke  (i)  the  sensation  of  light  and  its  different 
qualities — colors;  (2)  the  perception  of  light  and  color  under  the  form  of 
pictures  of  external  objects;  and  (3)  in  connection  with  the  ocular  muscles, 
the  production  of  muscle  sensations  by  which  the  size,  distance,  and  direc- 
tion of  objects  may  be  judged. 

The  specific  physiologic  stimulus  to  the  terminal  end-organ,  the  retina, 
is  the  impact  of  ether  vibrations.  In  general,  it  may  be  said  that,  at 
least  for  the  same  color,  the  intensity  of  the  objective  vibration  determines 
the  intensity  of  the  sensation. 

The  Eyeball. — The  eyeball,  or  organ  of  vision,  is  situated  at  the  fore 
part  of  the  orbital  cavity  and  is  supported  by  a  cushion  of  fat;  it  is  pro- 
tected from  injury  by  the  bony  walls  of  the  cavity,  the  lids,  and  the  lashes, 
and  is  so  situated  as  to  permit  of  an  extensive  range  of  vision.  The  eyeball 
is  loosely  held  in  position  by  a  fibrous  membrane,  the  capsule  of  Tenon, 
which  is  attached  on  the  one  hand  to  the  eyeball  itself  and  on  the  other  to 
the  walls  of  the  cavity.  Thus  suspended,  the  eyeball  is  capable  of  being 
moved  in  any  direction  by  the  contraction  of  the  muscles  attached  to  it. 


SENSE   OF   SIGHT  227 

Structure. — The  eyeball  is  spheroid  in  shape  and  measures  about  twenty- 
four  mm.  in  its  anteroposterior  diameter,  and  a  little  less  in  its  transverse 
diameter.  When  viewed  in  profile,  it  is  seen  to  consist  of  the  segments 
of  two  spheres,  of  which  the  posterior  is  the  larger,  occupying  five  sixths, 
and  the  anterior  the  smaller,  occupying  one  sixth,  of  the  ball. 

The  eye  is  made  up  of  several  membranes,  concentrically  arranged, 
within  which  are  inclosed  the  refracting  media  essential  to  vision. 

The  membranes,  enumerating  them  from  without  inward,  are  as  follows: 
the  sclera  and  cornea,  the  choroid,  iris  and  ciliary  muscle,  and  the  retina. 
The  refracting  media  are  the  aqueous  humor,  the  crystalline  lens,  and 
the  vitreous  humor. 

The  Sclera  and  Cornea. — The  sclera  is  the  opaque  fibrous  membrane 
covering  the  posterior  five  sixths  of  the  ball.  It  is  composed  of  connec- 
tive tissue  arranged  in  layers,  which  run  both  transversely  and  longitudin- 
ally; it  is  pierced  posteriorly  by  the  optic  nerve  about  J^o  o^  an  inch  inter- 
nal to  the  optic  axis.  The  sclera,  by  its  density  gives  form  to  the  eye  and 
protects  the  delicate  structures  within  it,  and  serves  for  the  attachment 
of  the  muscles  by  which  the  ball  is  moved. 

The  cornea  is  a  transparent  non- vascular  membrane  covering  the  ante- 
rior one  sixth  of  the  eyeball.  It  is  nearly  circular  in  shape  and  is  continu- 
ous at  the  circumference  with  the  sclera,  from  which  it  cannot  be  separated. 
The  substance  of  the  cornea  is  made  up  of  thin  layers  of  delicate,  trans- 
parent fibrils  of  connective  tissue,  more  or  less  united;  between  these 
layers  are  found  a  number  of  intercommunicating  lymph-spaces,  lined  by 
endothelium,  which  are  in  connection  with  lymphatics.  Leukocytes  or 
lymph-corpuscles  are  often  found  in  these  spaces.  At  the  junction  of 
the  cornea  and  sclera  there  is  a  circular  groove,  the  canal  of  Schlemm. 

The  Choroid,  the  Iris  and  the  Ciliary  Muscle. — These  three  structures 
together  constitute  the  second  or  middle  coat  of  the  eyeball. 

The  choroid  is  a  dark  brown  membrane  which  extends  forward  nearly  to 
the  cornea,  where  it  terminates  in  a  series  of  folds,  the  ciliary  processes. 
In  its  structure  the  choroid  is  highly  vascular,  consisting  of  both  arteries 
and  veins.  Externally  it  is  connected  with  the  sclerotic  by  connective 
tissue;  internally  it  is  lined  by  a  layer  of  hexagonal  pigment  cells,  which, 
though  usually  classed  as  belonging  to  the  choroid,  is  now  known  to  belong, 
embryologically  and  physiologically,  to  the  retina. 

The  choroid  with  its  contained  blood-vessels  bears  an  important  rela- 
tion to  the  nutrition  of  the  eye;  it  provides  for  the  blood-supply  and  for 
drainage  from  the  body  of  the  eye,  and  presents  a  uniform  and  high  tem- 
perature to  the  retina. 


2  28  HUMAN   PHYSIOLOGY 

The  iris  is  the  circular  variously  colored  membrane  placed  in  the  ante- 
rior portion  of  the  eye  just  behind  the  cornea.  It  is  perforated  a  little  to 
the  nasal  side  of  the  center  by  a  circular  opening,  the  pupil.  The  outer 
or  circumferential  border  is  connected  with  the  cornea,  ciliary  muscle,  and 
ciliary  processes;  the  free  inner  edge  forms  the  boundary  of  the  pupil,  the 
size  of  which  is  constantly  changing.  The  framework  of  the  iris  is  com- 
posed of  connective-tissue  blood-vessels,  muscle-fibers  and  pigmented 
connective-tissue  corpuscles.  The  anterior  surface  is  covered  with  a  layer 
of  epithelial  cells  continuous  with  those  covering  the  posterior  surface  of 
the  cornea;  the  posterior  surface  is  lined  by  a  limiting  membrane  bearing 
pigment  epithelial  cells  continuous  with  those  of  the  choroid.  The  vari- 
ous colors  which  the  iris  assumes  in  different  individuals  depend  upon  the 
quantity  and  disposition  of  the  pigment  granules. 

The  muscle-fibers  of  the  iris,  which  are  of  the  non-striated 'variety,  are 
arranged  in  two  sets — sphincter  and  dilatator. 

The  sphincter  pupillce  is  a  circular,  flat  band  of  muscle-fibers  surround- 
ing the  pupil  close  to  its  posterior  surface;  by  its  contraction  and  relaxa- 
tion the  pupil  is  diminished  or  increased  in  size.  The  dilatator  pupillcB  con- 
sists of  a  thin  layer  of  fibers  arranged  in  a  radiate  manner;  at  the  margin 
of  the  pupil  they  blend  with  those  of  the  sphincter  muscle,  while  at  the 
outer  border  they  arch  to  form  a  circular  muscular  layer. 

The  ciliary  muscle  is  a  gray,  circular  band,  consisting  of  unstriped 
muscle-fibers  about  Jfo  of  an  inch  long  running  from  before  backward. 
It  is  attached  anteriorly  to  the  inner  surface  of  the  sclera  and  cornea, 
and  posteriorly  to  the  choroid  coat  opposite  the  ciliary  processes.  At  the 
anterior  border  of  the  radiating  fibers  and  internally  are  found  bundles  of 
circular  muscle-fibers,  constituting  the  annular  muscle  of  Miiller.  The 
ciliary  muscle  thus  consists  of  two  sets  of  fibers,  a  radiating  and  a  circular, 
both  of  which  are  concerned  in  effecting  a  change  in  the  convexity  of  the 
lens  in  the  accommodation  of  the  eye  to  near  vision. 

The  retina  forms  the  internal  coat  of  the  eye.  In  the  fresh  state  it  is  a 
delicate,  transparent  membrane  of  a  pink  color,  but  after  death  soon  be- 
comes opaque;  it  extends  forward  almost  to  the  ciliary  processes,  where  it 
terminates  in  an  indented  border,  the  ora  serrata.  In  the  posterior  part 
of  the  retina,  at  a  point  corresponding,to  the  axis  of  vision,  is  a  yellow  spot, 
the  macula  lutea,  which  is  somewhat  oval  in  shape  and  tinged  with  yellow 
pigment.  It  presents  in  its  center  a  depression,  the  fovea  centralis,  corre- 
sponding to  a  decrease  in  thickness  of  the  retina;  about  Ho  of  an  inch  to 
the  inner  side  of  the  macula  is  the  point  of  entrance  of  the  optic  nerves. 
The  arteria  centralis  retincp  pierces  the  optic  nerve  near  the  sclerotic,  runs 


SENSE    OF   SIGHT 


229 


forward  in  its  substance,  and  is  distributed  in  the  retina  as  far  forward  as 
the  ciliary  processes. 

The  retina  is  remarkably  complex  consisting  of  ten  distinct  layers 
from  without  inward.  For  physiologic  purposes  they  may  be  resolved 
into  three — viz.: 

1.  The  layer  of  visual  cells,  the  rods  and  cones. 

2.  The  layer  of  bipolar  cells. 

3.  The  layer  of  ganglionic  cell.     Fig.  23. 
The  number  of  optic  nerve-fibers  in  the  retina  is 

estimated  to  be  about  800,000,  and  for  each  fiber 
there  are  about  seven  cones,  one  hundred  rods,  and 
seven  pigment  cells.  The  points  of  the  rods  and 
cones  are  directed  toward  the  choroid,  or  away  from 
the  entering  light,  and  dip  into  the  pigment  layer. 
They,  with  the  pigment  layer,  are  the  intermediat- 
ing elements  in  the  change  of  the  etheral  vibra- 
tions into  nerve  force;  out  of  these  nerve  vibrations 
the  brain  fashions  the  sensations  of  light,  form,  and 
color. 


Fig. 


23. — Retinal 
Cells. 


5',  z' .  Visual  cells 
with  their  peripheral 
terminations,  s.  Rods, 
z.  Cones,  b.  Bipolar 
cells,  g.  Ganglion  cells 
from  which  arise  the 
axons  of  the  optic 
nerve. 


The  Refracting  Media. — The  vitreous  humor, 
which  supports  the  retina,  is  the  largest  of  the  re- 
fracting media;  it  is  globular  in  form  and  constitutes 
about  four-fifths  of  the  ball,  it  is  hollowed  out  ante- 
riorly for  the  reception  of  the  crystalline  lens.  The 
outer  surface  of  the  vitreous  is  covered  by  a  delicate, 
transparent  membrane,  termed  the  hyaloid,  mem- 
brane, which  serves  to  maintain  its  globular  form. 

The  aqueous  humor,  found  in  the  anterior  chamber  of  the  eye,  is  a  clear 
alkaline  fluid,  having  a  specific  gravity  of  1003- 1009.  It  is  secreted  most 
probably  by  the  blood-vessels  of  the  iris  and  ciliary  processes.  It  passes 
from  the  interior  of  the  eye,  through  the  canal  of  Schlemm  and  the  meshes 
at  the  base  of  the  iris,  into  the  lymph  vessels  and  thus  increased  ocular 
tension  is  prevented. 

The  crystalline  lens,  inclosed  within  its  capsule,  is  a  transparent  biconvex 
body,  situated  just  behind  the  iris  and  resting  in  the  depression  in  the 
anterior  part  of  the  vitreous.  The  two  convexities  are  not  quite  alike, 
the  curvature  of  the  posterior  surface  being  slightly  greater  than  that  of 
the  anterior.  The  lens  measures  about  }^  of  an  inch  in  the  transverse 
diameter  and  J^  of  an  inch  in  the  anteroposterior  diameter. 

The  suspensory  ligament,  by  which  the  lens  is  held  in  position,  is  a  firm, 


230 


HUMAN  PHYSIOLOGY 


Fig.  24. — Sclerotic  Coat  Removed  to  Show  Choroid  Ciliary  Muscle,  and 
Nerves. — (Holden.) 
fl.f Sclerotic  coat.     b.  Veins  of  the  choroid,     c.  Ciliary  nerves,     d.  Veins  of  the 
choroid,     e.  Ciliary  muscle.    /.  Iris. 


Fig.  25. — Diagram  of  a  Vertical  Section  of  the  Eye. — (Holden.) 
I.  Anterior  chamber  filled  with  aqueous  humor.     2.  Posterior  chamber.     3.  Cana 
of  Petit,     o.  Hyaloid  membrane,     b.  Retina  (dotted  line;,     c.  Choroid  coat  (black 
line),     d.  Sclerotic  coat.     e.  Cornea.    /.  Iris.     g.  Ciliary  processes,     h.  Canal  of 
Schlemn  or  Fontana.     i.  Ciliary  muscle. 


PHYSIOLOGY   or  VISION  23 1 

transparent  membrane,  united  to  the  ciliary  processes.  A  short  distance 
beyond  its  origin  it  splits  into  two  layers,  the  anterior  of  which  is  inserted 
into  the  capsule  of  the  lens  and  blends  with  it;  the  posterior,  passing 
inward  behind  the  lens,  becomes  united  to  its  capsule.  The  anterior 
layer  presents  a  series  of  foldings,  zone  of  Zinn,  which  are  inserted  into  the 
intervals  of  the  folds  of  the  ciliary  processes.  The  triangular  space 
between  the  two  layers  is  the  canal  of  Petit. 

Blood-vessels  and  Nerves. — The  structures  composing  the  eyeball  are 
supplied  with  blood  by  the  long  and  short  ciliary  arteries,  branches  of  the 
opthalmic;  they  pierce  the  sclerotic  at  various  points  and  are  ultimately 
distributed  to  all  tissues  within  the  ball. 

The  nerves  distributed  to  the  non-striated  muscles  of  the  eyeball — the 
ciliary  muscle  and  the  sphincter  muscle  of  the  iris — are  postganglionic 
fibers  coming  from  the  ciliary  or  ophthalmic  ganglion;  distributed  to  this 
ganglion  are  preganglionic  fibers  coming  from  the  central  nerve  system 
through  the  oculo-motor  nerve.  The  nerves  distributed  to  the  dilatator 
muscle  of  the  iris  and  to  the  blood-vessels  are  postganglionic  fibers  com- 
ing from  the  superior  cervical  ganglion;  distributed  to  this  ganglion  are 
preganglionic  fibers  coming  from  the  central  nerve  system  through  the 
upper  thoracic  nerves  and  the  cervical  cord  of  the  sympathetic.  Sensory 
nerves  are  derived  from  the  trigeminal.  The  relationship  of  the  structures 
composing  the  eyeball  is  shown  in  Figs.  24,  25. 

THE  PHYSIOLOGY  OF  VISION 

The  Retinal  Image. — The  general  function  of  the  eye  is  the  formation 
of  images  of  external  objects  on  the  free  ends  of  the  percipient  elements  of 
the  retina,  the  rods  and  cones.  The  existence  of  an  image  on  the  retina 
can  be  readily  seen  in  the  excised  eye  of  an  albino  rabbit,  when  placed 
between  a  lighted  candle  and  the  eye  of  an  observer.  Its  presence  in  the 
human  eye  can  be  demonstrated  with  the  ophthalmoscope.  It  is  this 
image,  composed  of  focal  points  of  luminous  rays,  that  stimulate  the  rods 
and  cones,  which  is  the  basis  of  our  sight  perceptions,  and  out  of  which  the 
mind  constructs  space  relations  of  external  objects.  Whatever  the  dis- 
tance, the  image  is  generally  smaller  than  the  object;  it  is  also  reversed, 
the  upper  part  of  the  object  becoming  the  lower  part  of  the  image,  and  the 
right  side  of  the  object  the  left  side  of  the  image. 

The  Dioptric  or  Refracting  Apparatus. — The  formation  of  an  image  is 
made  possible  by  the  introduction  of  a  complex  refracting  apparatus  con- 
sisting of  the  cornea,  aqueous  humor,  lens,  and  vitreous  humor.    Without 


232  HUMAN  PHYSIOLOGY 

these  agencies  the  ether  vibrations  would  give  rise  only  to  a  sensation  of 
diffused  luminosity.  Rays  of  light  emanating  from  any  one  point  arriv- 
ing at  the  eye  must  traverse  successively  the  different  refracting  media. 
In  their  passage  from  one  to  the  other,  they  undergo  at  their  surfaces 
changes  in  direction  before  they  are  finally  converged  to  a  focal  point  on 
the  retina. 

Inasmuch  as  the  two  surfaces  of  the  cornea  are  parallel  and  its  refractive 
power  practically  the  same  as  the  aqueous  humor,  the  media  may  be 
reduced  to  three — viz. : 

1.  Cornea  and  aqueous  humor, 

2.  The  lens. 

3.  The  vitreous  humor. 

The  refracting  surfaces  may  also  be  reduced  to  three — viz.: 

1.  Anterior  surface  of  the  cornea. 

2.  Anterior  surface  of  lens. 

3.  Posterior  surface  of  lens. 

The  refraction  effected  by  the  cornea  is  very  great,  owing  to  the  passage 
of  the  light  from  the  air  into  a  comparatively  dense  medium,  and  is  suffi- 
cient of  itself  to  bring  parallel  rays  of  light  to  a  focus  about  ten  millimeters 
behind  the  retina.  This  would  be  the  condition  in  an  eye  in  which  the 
lens  was  congenitally  absent  or  after  removal  by  surgical  procedures. 
Perfect  vision  requires,  however,  that  the  convergence  of  the  light  shall 
be  great  enough  to  allow  the  image  to  fall  upon  the  retina.  This  is  accom- 
plished in  part  by  the  crystalline  lens,  a  body  denser  than  the  cornea  and 
possessing  a  higher  refractive  power.  After  passing  through  the  lens  the 
rays  of  light  if  continued  would  come  to  a  focus  about  6.5  mm.  behind  the 
retina.  On  passing  from  the  lens  into  the  vitreous — i.e.,  from  a  denser 
into  a  rarer  medium — the  rays  are  once  more  converged  and  to  an  extent 
sufficient  to  focalize  them  on  the  retina.  The  function  of  the  cornea  and 
lens  is  to  focalize  the  rays  with  the  production  of  an  image. 

The  Visual  Angle. — The  visual  angle  is  defined  as  the  angle  formed  by 
the  intersection  of  two  lines  drawn  from  the  extremities  of  an  object  to 
the  nodal  point  of  the  eye  which  lies  near  the  posterior  surface  of  the  lens 
about  15.5  millimeters  from  the  retina.  Beyond  the  nodal  point,  however, 
the  lines  again  diverge  and  form  an  inverted  or  reversed  image  of  the 
object  on  the  retina.  The  size  of  the  visual  angle  increases  with  the  near- 
ness and  decreases  with  the  remoteness  of  the  object;  the  retinal  image 
correspondingly  increases  and  decreases  in  size. 

The  Size  of  the  Retinal  Image. — The  size  of  the  retinal  image  depends 
upon  the  visual  angle,  which  in  turn  depends  upon  the  size  of  the  object 


PHYSIOLOGY  OF  VISION  233 

and  its  distance  from  the  eye.  At  a  distance  of  15.2596  meters  the  image 
of  an  object  one  meter  high  would  be  one  millimeter,  or  a  thousand  times 
smaller  than  the  object. 

The  size  of  the  image  may  be  calculated  from  the  following  equation. 
The  size  of  the  object  is  to  the  size  of  the  image,  as  the  distance  of  the 
object  from  the  nodal  point,  is  to  the  distance  of  the  nodal  point  from  the 
retina.  (The  distance  of  the  nodal  point  from  the  anterior  surface  of 
the  cornea  is  7.3  mm.) 

Accommodation. — By  accommodation  is  understood  the  power  which 
the  eye  possesses  of  adjusting  itself  to  vision  at  different  distances.  In  a 
normal  or  emmetropic  eye  parallel  rays  of  light  are  brought  to  a  focus  on 
the  retina;  but  divergent  rays — that  is,  rays  coming  from  a  near  luminous 
point — will  be  brought  to  a  focus  behind  the  retina,  provided  the  refrac- 
tive media  remain  the  same;  as  a  result,  vision  would  be  indistinct,  from 
the  formation  of  diffusion  circles.  It  is  impossible  to  see  distinctly,  there- 
fore, a  near  and  a  distant  object  at  the  same  time.  We  must  alternately 
direct  the  vision  from  one  to  the  other.  A  normal  eye  does  not  require 
adjusting  for  parallel  rays;  but  for  divergent  rays  a  change  in  the  eye  is 
necessitated;  this  is  termed  accommodation.  In  the  accommodation  for 
near  vision  the  lens  becomes  more  convex,  particularly  on  its  anterior 
surface.  The  increase  in  convexity  augments  its  refractive  power;  the 
greater  the  degree  of  divergence  of  the  rays  previous  to  entering  the 
eye,  the  greater  the  increase  of  convexity  of  the  lens  and  convergence  of 
the  rays  after  passing  through  it.  By  this  alteration  in  the  shape  of  the 
lens  we  are  enabled  to  focus  light  rays  coming  from,  and  to  see  distinctly, 
near  as  well  as  distant  objects. 

Function  of  the  Ciliary  Muscle. — Though  it  is  admitted  that  the  change 
in  the  convexity  of  the  lens  is  caused  by  the  contraction  of  the  ciliary 
muscle  and  the  relaxation  of  the  suspensory  ligament,  the  exact  manner 
.in  which  it  does  so  is  not  understood.  When  the  eye  is  in  repose,  as  in 
distant  vision,  the  suspensory  ligament  is  tense,  and  the  lens  possesses  that 
degree  of  curvature  necessary  for  focusing  parallel  rays.  In  the  voluntary 
efforts  to  accommodate  the  eye  for  near  vision,  the  ciliary  muscle  con- 
tracts, the  suspensory  ligament  relaxes,  and  the  lens,  inherently  elastic, 
bulges  forward  and  once  again  focuses  the  rays  upon  the  retina.  It  is,' 
therefore,  termed  the  muscle  of  accommodation,  and  by  its  alternate 
contraction  and  relaxation  the  lens  is  rendered  more  or  less  convex, 
according  to  the  requirements  for  near  and  distant  vision. 

Range  of  Accommodation. — Parallel  rays  coming  from  a  luminous  point 
distant  not  less  than  200  feet  do  not  require  adjustment;  from  this  point 


234  HUMAN  PHYSIOLOGY 

up  to  infinity  no  accommodation  is  required  for  perfect  vision.  This  is 
termed  the  punctum  remotum,  and  indicates  the  distance  to  which  an 
object  may  be  removed  and  yet  distinctly  seen.  If  the  object  be  brought 
nearer  to  the  eye  than  200  feet,  the  accommodative  power  must  come  into 
play;  the  nearer  the  object,  the  more  energetic  must  be  the  contraction 
of  the  ciliary  muscle  and  the  consequent  increase  in  the  convexity  of  the 
lens.  At  a  distance  of  five  inches,  however,  the  power  of  accommodation 
reaches  its  maximum;  this  is  termed  the  punctum  proximum^  and  indicates 
the  nearest  point  at  which  an  object  may  be  seen  distinctly.  The 
distance  between  these  two  points  is  the  range  of  accomniodation. 

The  Function  of  the  Iris. — The  iris  plays  the  part  of  a  diaphragm,  and 
by  means  of  its  central  aperture  the  pupil  regulates  the  quantity  of 
light  entering  the  interior  of  the  eye;  by  preventing  rays  from  passing 
through  the  margin  of  the  lens  it  diminishes  spheric  aberration.  The  size 
of  the  pupil  depends  upon  the  relative  degree  of  contraction  of  the  circular 
and  radiating  fibers;  the  variations  in  size  of  the  pupil  from  variations 
in  the  degree  of  contraction  depend  upon  different  intensities  of  light.  If 
the  light  be  intense,  the  circular  fibers  contract,  and  diminish  the  size  of 
the  pupil;  if  the  light  diminishes  in  intensity,  the  circular  fibers  relax  and 
the  pupil  enlarges,  | 

Point  of  Most  Distinct  Vision. — While  all  portions  of  the  retina  are 
sensitive  to  light,  their  sensibility  varies  within  wide  limits.  At  the 
macula  lutea,  and  more  especially  in  its  most  central  depression,  the  fovea, 
where  the  retinal  elements  are  reduced  practically  to  the  layer  of  rods  and 
cones,  the  sensibility  reaches  its  maximum.  It  is  at  this  point  that  the 
image  is  found  when  vision  is  most  distinct.  The  macula  and  fovea  are 
always  in  the  line  of  direct  vision.  From  the  macula  toward  the  periphery 
of  the  retina  there  is  a  gradual  diminution  in  sensibility,  and  a  corre- 
sponding decline  in  the  distinctness  of  vision.  In  those  portions  of  the 
retina  lying  outside  the  macula,  the  indistinctness  of  vision  depends  not 
only  on  diminished  sensibility,  but  also  upon  inaccurate  focusing  of  the 
rays. 

Blind  Spot. — Although  the  optic  nerve  transmits  the  impulses  excited 
in  the  retina  by  the  ethereal  vibration,  the  nerve-fibers  themselves  are 
insensitive  to  light.  At  the  point  of  entrance  of  the  optic  nerve,  owing  to 
the  absence  of  the  rods  and  cones,  the  rays  of  light  make  no  impression. 
This  is  the  blind  spot.  As  this  spot  is  not  in  the  line  of  vision,  no  dark 
point  is  ordinarily  observed  in  the  field  of  vision — the  circular  space  before 
a  fixed  eye  within  which  reflections  of  objects  are  perceptible. 

The  rods  and  cones  are  the  most  sensitive  portions  of  the  retina.     A  ray 


OPTIC  DEFECTS  235 

of  light  entering  the  eye  passes  entirely  through  the  various  layers  of  the 
retina,  and  is  arrested  only  upon  reaching  the  pigmentary  epithelium  in 
which  the  rods  and  cones  are  embedded.  As  to  the  manner  in  which  the 
objective  stimuli — light  and  color,  so  called — are  transformed  into  nerve 
impulses,  but  little  is  known.  It  is  probable  that  the  ethereal  vibrations 
are  transformed  into  heat,  which  excites  the  rods  and  cones.  These,  act- 
ing as  highly  specialized  end  organs  of  the  optic  nerve,  start  the  impulses 
on  their  way  to  the  brain,  where  the  seeing  process  takes  place.  As  to 
the  relative  function  of  the  rods  and  cones,  it  has  been  suggested,  from  the 
study  of  the  facts  of  comparative  anatomy,  that  the  rods  are  impressed 
only  by  differences  in  the  intensity  of  light,  while  the  cones,  in  addition, 
are  impressed  by  qualitative  differences  or  color. 

The  Eyeball  a  Living  Camera  Obscura. — The  eyeball  may  be_compared 
in  a  general  wa>  to  a  camera  obscura.  The  anatomic  arrangement  of  its 
structures  reveals  many  points  of  similarity.  The  sclerotic  and  choroid 
may  be  compared  with  the  walls  of  the  chamber;  the  combined  refractive 
media,  cornea,  aqueous  humor,  lens,  and  vitreous  humor,  to  the  lens  for 
focusing  the  rays  of  light;  the  retina,  to  the  sensitive  plate  receiving  the 
image  formed  at  the  focal  point;  the  iris,  to  the  diaphragm,  which,  by 
cutting  off  the  marginal  rays,  prevents  spheric  aberration  and  at  the  same 
time  regulates  the  amount  of  light  entering  the  eye;  the  ciliary  muscle, 
to  the  adjusting  screw,  by  which  distinct  images  are  thrown  upon  the 
retina  in  spite  of  varying  distances  of  the  object  from  which  the  light  rays 
emanate. 

OPTIC  DEFECTS 

Presbyopia. — Presbyopia  may  be  defined  as  a  condition  of  the  normal 
eye  in  which  the  accommodation  has  become  so  reduced  b>  age  that  read- 
ing has  become  impossible  at  ordinary  distances.  As  age  advances  the 
lens  gradually  loses  its  elasticity  and  hence  its  power  to  increase  in  con- 
vexity and  thickness  to  the  same  extent  as  in  earlier  life,  in  response  to 
efforts  of  accommodation.  The  refractive  power  is,  thereby,  lessened 
and  the  eye  is  no  longer  able  to  see  distinctly  at  the  normal  reading  dis- 
tances, viz.:  22  to  28  cm.  Rays  of  light  emanating  from  a  luminous  point 
at  the  normal  reading  distances  are  less  and  less  converged  on  the  retina 
and  hence  the  diffusion  circles  increase  in  size.  The  near  point,  the  point 
from  which  divergent  rays  can  be  focalized,  therefore  advances  toward 
the  far  point,  or  recedes^^from  the  individual.  The  range  of  accommoda- 
tion is,  thereby,  diminished. 


236  HUMAN  PHYSIOLOGY 

Myopia. — Myopia  may  be  defined  as  a  condition  of  the  eye  character- 
ized by  an  increase  in  the  antero-posterior  diameter  or  by  a  hypernormal 
refracting  power  of  the  lens.  The  former  is  the  usual  condition.  In 
either  case  parallel  rays  of  light  which  enter  the  eye  are  brought  to  a  focus 
in  front  of  the  retina  after  which  they  diverge  and  give  rise  to  diffusion 
circles  and  indistinctness  of  vision.  Divergent  rays,  however,  which  enter 
the  eye  are  focalized  as  usual  on  the  retina  even  in  its  new  position.  The 
distant  point,  the  punctum  remotum,  is  always  at  a  finite  distance,  but 
approaches  the  eye  as  the  myopia  increases.  The  near  point  is  usually 
much  nearer  the  eye  than  20  cm.  For  this  reason  the  condition  is  termed 
near  sight. 

Hypermetropia. — Hypermetropia  may  be  defined  as  a  condition  of  the 
eye  characterized  by  decrease  of  the  normal  antero-posterior  diameter  or 
by  a  subnormal  refracting  power  of  the  lens.  The  former  is  the  usual  con- 
dition. In  either  case  parallel  rays  of  light  which  enter  the  eye  are,  there- 
fore, not  brought  to  a  focus  when  the  accommodation  is  suspended.  Fall- 
ing on  the  retina  previous  to  focalization,  they  give  rise  to  diffusion- 
circles  and  indistinctness  of  the  image.  As  no  object  can  be  seen  distinctly 
no  matter  how  remote,  there  is  no  positive  far  point.  The  near  point  is 
abnormally  distant — sometimes  as  far  as  200  cm.  For  this  reason  the 
condition  is  termed  far  sight.  A  hypermetropic  eye  without  accommoda- 
tive effort  can  focalize  only  converging  rays  on  the  retina. 

Astigmatism. — Astigmatism  may  be  defined  as  a  condition  of  the  eye 
characterized  by  an  inequality  of  curvature  of  its  refracting  surfaces  in 
consequence  of  which  not  all  of  the  rays  coming  from  a  single  point  are 
brought  to  the  same  focus.  The  inequality  may  be  either  in  the  cornea 
or  lens,  or  both,  though  usually  in  the  cornea. 

In  the  normal  cornea  the  radius  of  curvature  in  the  vertical  meridian 
is  a  trifle  shorter,  7.6  mm.,  than  that  of  the  horizontal,  7.8  mm.,  and 
hence  its  focal  distance  is  slightly  shorter.  The  difference,  however,  in 
the  focal  distances  is  so  slight  that  the  error  in  the  formation  of  the 
image  is  scarcely  noticeable.  A  transverse  section  of  a  cone  of  light  com- 
ing from  the  cornea  is  practically  a  circle.  If,  however,  the  vertical 
curvature  exceeds  the  normal  to  any  marked  extent,  the  rays  passing  in 
the  vertical  plane  will  be  more  sharply  refracted  and  brought  to  a  focus 
much  sooner  than  the  rays  passing  through  the  horizontal  plane.  The 
result  will  be  that  the  cone  of  light  will  be  no  longer  circular,  but  more 
or  less  elliptic.  Though  the  vertical  plane  has  usually  the  sharper  curva- 
ture, it  not  infrequently  happens  that  the  reverse  is  true.     For  the  reason 


SENSE   OF   HEARING  237 

that  the  rays  from  one  point  do  not  all  come  to  the  same  focus  or  point, 
the  condition  is  termed  astigmatism. 

Movements  of  the  Eyeball. — The  almost  spheric  eyeball  lies  in  the 
correspondingly  shaped  cavity  of  the  orbit,  like  a  ball  placed  in  a  socket, 
and  is  capable  of  being  rotated  to  a  considerable  extent  by  the  six  muscles 
which  are  attached  to  it.  These  muscles  are  the  superior  and  inferior 
recti,  the  external  and  internal  recti,  and  the  superior  and  inferior  obliqui. 
The  four  recti  muscles  arise  from  the  apex  of  the  orbit  cavity,  from  which 
point  they  pass  forward  to  be  inserted  into  the  sclera  about  7  to  8  mm. 
from  the  corneal  border.  The  superior  oblique  muscle  having  a  similar 
origin  passes  forward  to  the  upper  and  inner  angle  of  the  orbit  cavity, 
at  which  point  its  tendon  passes  through  a  cartilaginous  pulley,  after 
which  it  is  reflected  backward  to  be  inserted  into  the  superior  surface  of 
the  sclera  about  16  mm.  behind  the  corneal  border.  The  inferior  oblique 
muscle  arises  from  the  inner  and  inferior  angle  of  the  orbit  cavity.  It 
then  passes  outward,  upward,  and  backward,  to  be  inserted  into  the 
upper,  posterior,  and  temporal  portion  of  the  sclera  about  4  or  5  mm. 
from  the  optic  nerve  entrance. 

The  superior  and  inferior  recti  muscles,  forming  one  pair,  move  the  eye 
around  a  horizontal  axis  which  intersects  the  median  plane  of  the  body 
in  front  of  the  eyes  at  an  angle  of  63  degrees;  the  external  and  internal 
recti  muscles,  forming  a  second  pair,  move  the  eye  around  a  vertical 
axis;  the  superior  and  inferior  oblique  muscles  forming  the  third  pair 
rotate  the  globe  around  a  horizontal  axis  which  cuts  the  median  plane  of 
the  body  behind  the  eyes  at  an  angle  of  39  degrees.  Thus  it  is  that  each 
muscle  moves  the  eye  as  follows,  the  movement  for  practical  purposes 
being  referred  to  the  cornea:  The  rectus  externus  draws  the  cornea 
simply  to  the  temporal  side,  the  rectus  internus  simply  to  the  nose;  the 
superior  rectus  displaces  the  cornea  upward,  slightly  inward,  and  turns 
the  upper  part  toward  the  nose  (medial  torsion) ;  the  inferior  rectus  move 
the  cornea  downward,  slightly  inward,  and  twists  the  upper  part  away 
from  the  nose  (lateral  torsion);  the  superior  oblique  displaces  the  cornea 
downward,  slightly  outward,  and  produces  medial  torsion;  while  the 
inferior  oblique  moves  the  cornea  upward,  slightly  outward,  and  produces 
lateral  torsion.  These  facts  show  that  for  certain  movements  of  the 
eye  at  least  three  muscles  are  necessary. 

THE  SENSE  OF  HEARING 

The  physiologic  mechanism  involved  in  the  sense  of  hearing  includes 
the  ear,  the  acoustic  nerve,  the  acoustic  tract  (the  lateral  fillet  or  lem- 


238  HUMAN  PHYSIOLOGY 

niscus),  the  acoustic  radiation,  and  nerve-cells  in  the  thorax  of  the 
temporal  lobe. 

Peripheral  stimulation  of  this  mechanism  develops  nerve  impulses 
which,  transmitted  to  the  cerebral  cortex,  evoke  the  sensation  of  sound 
and  its  varying  qualities — intensity,  pitch,  and  timbre. 

The  specific  physiologic  stimulus  to  the  terminal  organ,  the  organ  of 
Corti,  is  the  impact  of  atmospheric  pulsations  of  varying  energy  and 
rapidity. 

The  ear,,  or  organ  of  hearing,  is  lodged  within  the  petrous  portion  of 
the  temporal  bane.  It  may  be,  for  convenience  of  description,  divided 
into  three  portions — viz. : 

1.  The  external  ear. 

2.  The  middle  ear. 

3.  The  internal  ear  or  labyrinth. 

The  External  Ear. — The  external  ear  consists  of  the  pinnay  or  auricle^ 
and  the  external  atiditory  canaL  The  pinna  consists  of  a  thin  layer  of 
cartilage,  presenting  a  series  of  elevations  and  depressions;  it  is  attached 
by  fibrous  tissue  to  the  outer  bony  edge  of  the  auditory  canal;  it  is  covered 
by  a  layer  of  integument  continuous  with  that  covering  the  side  of  the 
head.  The  general  shape  of  the  pinna  is  concave,  and  presents,  a  little 
below  the  center,  a  deep  depression — the  concha.  The  external  auditory 
canal  extends  from  the  concha  inward  for  a  distance  of  about  i  J^  inches. 
It  is  directed  somewhat  forward  and  upward,  passing  over  a  convexity 
of  bone,  and  then  dips  downward  to  its  termination;  it  is  composed  of 
both  bone  and  cartilage,  and  is  lined  with  a  reflection  of  the  skin  covering 
the  pinna.  At  the  external  portion  of  the  canal  the  skin  contains  a  number 
of  tubular  glands — The  ceruminous  glands — which  in  their  conformation 
resemble  the  perspiratory  glands.    They  secrete  the  cerumen,  or  ear-wax. 

The  Middle  Ear. — The  middle  ear,  or  tympanum,  is  an  irregularly 
shaped  cavity  hollowed  out  of  the  temporal  bone  and  situated  between 
the  external  ear  and  the  labyrinth.  It  is  narrow  from  side  to  side^  but 
relatively  long  in  its  vertical  and  anteroposterior  diameters;  it  is  separated 
from  the  external  auditory  canal  by  a  membrane — the  memhrana  tympani; 
from  the  internal  ear  it  is  separated  by  an  osseo-membranous  partition, 
which  forms  a  common  wall  for  both  cavities.  The  middle  ear  communi- 
cates posteriorly  with  the  mastoid  cells;  anteriorly  with  the  nasopharynx 
by  means  of  the  Eustachian  tube.  The  interior  of  this  cavity  is  lined  by 
mucous  membrane  continuous  with  that  lining  the  pharnyx  (Fig.  26). 

The  Membrana  Tympani. — The  membrana  tympani  is  a  thin,  trans- 
lucent, nearly  circular  membrane,  measuring  about  %  of  an  inch  in  di- 


SENSE  OF  HEARING  239 

ameter,  placed  at  the  inner  termination  of  the  external  auditory  canal. 
The  membrane  is  inclosed  within  a  ring  of  bone,  which  in  the  fetal  condi- 
tion can  be  easily  removed,  but  in  the  adult  condition  becomes  consoli- 
dated with  the  surrounding  bone.  The  membrana  tympani  consists 
primarily  of  a  layer  of  fibrous  tissue,  arranged  both  circularly  and  radially, 
and  forms  the  membrana  propria;  externally  it  is  covered  by  a  thin  layer 
of  skin  continuous  with  that  lining  the  auditory  canal;  internally  it  is 


Iffiii^ 


Fig.  26. — Tympanum  and  Auditory  Ossicles  (Left)  Magnified. 
A.G.  External  meatus.  M.  Membrana  tympani,  which  is  attached  to  the  handle 
of  the  malleus,  n,  and  near  it  the  short  process,  p.  h.  Head  of  the  malleus,  a. 
Incus;  K.  its  short  process,  with  its  ligament:  L  long  process,  s.  Sylivian  ossicle. 
S.  Stapes.  Ax,  Ax,  is  the  axes  of  rotation  of  the  ossicles;  it  is  shown  in  perspective 
and  must  be  imagined  to  penetrate  the  plane  of  the  paper,  t.  Line  of  traction  of 
the  tensor  tympani.  The  other  arrows  indicate  the  movement  of  the  ossicles  when 
the  tensor  contracts. 

covered  by  a  thin  mucous  membrane.  The  tympanic  membrane  is 
placed  obliquely  at  the  bottom  of  the  auditory  canal,  inclining  at  a 
angle  of  forty-five  degrees,  being  directed  from  behind  and  above  down- 
ward and  inward.  On  its  external  surface  this  membrane  presents  a 
funnel-shaped  depression,  the  sides  of  which  are  somewhat  convex. 

The  Ear  Bones.— Running  across  the  tympanic  cavity  and  forming 
an  irregular  line  of  joined  levers  is  a  chain  of  bones  which  articulate  with 


240  HUMAN   PHYSIOLOGY 

one  another  at  their  extremities.  They  are  known  as  the  malleus,  incus, 
and  stapes. 

The  forms  and  position  of  these  bones  are  shown  in  figure  36. 

The  malleus  consists  of  a  head,  neck,  and  handle,  of  which  the  latter 
is  attached  to  the  inner  surface  of  the  membrana  tympani;  the  incus, 
or  anvil  bone,  presents  a  concave,  articular  surface,  which  receives  the 
head  of  the  malleus;  the  stapes,  or  stirrup  bone,  articulates  externally 
with  the  long  process  of  the  incus,  and  internally,  by  its  oval  base,  with 
the  edges  of  the  foramen  ovale. 

The  Tensor  Tympani. — The  tensor  tympani  muscle  consists  of  a  fleshy, 
tapering  portion,  Ji  of  an  inch  in  length,  which  terminates  in  a  slender 
tendon;  it  arises  from  the  cartilaginous  portion  of  the  Eustachian  tube 
and  the  adjacent  surface  of  the  sphenoid  bone.  From  this  origin  the 
muscle  passes  nearly  horizontally  backward  to  the  tympanic  cavity;  just 
opposite  to  the  fenestra  ovalis  its  tendon  bends  at  a  right  angle  over  the 
processus  cochleariformis,  and  then  passes  outward  across  the  cavity,  to 
be  inserted  into  the  angle  of  the  malleus  near  the  neck. 

The  Stapedius  Muscle. — The  stapedius  muscle  emerges  from  the  cavity 
of  a  pyramid  of  bone  projecting  from  the  posterior  wall  of  the  tympanum; 
the  tendon  passes  forward,  and  is  inserted  into  the  neck  of  the  stapes  bone 
posteriorly,  near  its  point  of  articulation  with  the  incus. 

The  Eustachian  Tube. — The  Eustachian  tube,  by  means  of  which  a  free 
communication  is  established  between  the  middle  ear  and  the  pharynx, 
is  partly  bone  and  partly  cartilaginous  in  structure.  It  measures  about 
1 3^  inches  in  length;  commencing  at  its  opening  into  the  nasopharynx,  it 
passes  upward  and  outward  to  the  spine  of  the  sphenoid  bone,  at  which 
point  it  becomes  somewhat  contracted;  the  tube  then  dilates  as  it  passes 
backward  into  the  middle-ear  cavity;  it  is  lined  by  mucous  membrane, 
which  is  continued  into  the  middle  ear  and  mastoid  cells. 

The  Function  of  the  Ear. — The  function  of  the  ear,  as  a  whole,  is  the 
reception  and  transmission  of  aerial  vibrations  to  the  terminal  organs 
concealed  within  the  internal  ear,  and  which  are  connected  with  the 
auditory  nerve-fibers.  The  excitation  of  these  end  organs  caused  by  the 
impact  of  the  vibration  arouses  in  the  auditory  nerve  impulses  which  are 
then  transmitted  to  the  brain,  where  the  hearing  process  takes  place. 
In  order  to  appreciate  the  functions  of  the  individual  parts  of  the  ear,  a 
few  of  the  characteristics  of  sound  waves  must  be  kept  in  mind. 

Sound  Waves. — All  sounds  are  caused  by  vibrations  in  the  atmosphere 
which  have  been  communicated  to  it  by  vibrating  elastic  bodies,  such  as 


SENSE   OF  HEARING  24I 

membranes,  strings,  rods,  etc.  These  vibrating  bodies  produce  in  the 
air  a  to-and-fro  movement  of  its  particles,  resulting  in  a  series  of  alternate 
condensations  and  rarefactions,  which  are  propagated  in  all  directions.. 
A  complete  oscillation  of  a  particle  of  air  forward  and  backward  consti- 
tutes a  sound  wave.  Musical  sounds  are  caused  by  a  succession  of  regular 
waves,  which  follow  one  another  with  a  certain  rapidity.  Noises  are 
caused  by  the  impact  of  a  series  of  irregular  waves. 

All  sound  waves  possess  intensity,  pitch,  and  equality.  The  intensity^ 
or  loudness,  of  a  sound  depends  upon  the  amplitude  of  its  vibrations  or 
on  the  extent  of  its  excursion.  The  pitch  depends  upon  the  number  of 
vibrations  which  affect  the  auditory  nerve  in  a  second  of  time;  the  pitch 
of  the  note  C,  the  first  below  the  leger  line  of  the  musical  scale,  is  caused 
by  256  vibrations  a  second;  the  pitch  of  the  same  note  an  octave  higher 
is  caused  by  512  vibrations  a  second.  If  the  vibrations  are  too  few  a 
second,  they  fail  to  be  perceived  as  a  continuous  sound;  the  minimum 
number  of  vibrations  capable  of  producing  a  sound  has  been  fixed  at 
sixteen  a  second;  the  highest  pitched  musical  note  capable  of  being  heard 
has  been  shown  to  be  due  to  38,000  vibrations  a  second.  In  the  ascent 
of  the  musical  scales  there  is,  therefore,  a  gradual  increase  in  the  number 
of  vibrations  and  a  gradual  increase  in  the  pitch  of  the  sounds.  Between 
the  two  extreme  limits  lies  the  range  of  audibility,  which  embraces  eleven 
octaves,  of  which  seven  are  employed  in  the  musical  scale. 

The  quality  of  sound  depends  upon  a  combination  of  the  fundamental 
vibration  with  certain  secondary  vibrations  of  subdivisions  of  the  vibrat- 
ing body.  These  so-called  over-tones  vary  in  intensity  and  pitch,  and 
by  modifying  the  form  of  the  primary  wave  produce  that  which  is  termed 
the  quality  of  sound. 

Function  of  the  Pinna  and  External  Auditory  Canal. — In  those  animals 
possessing  movable  ears  the  pinna  plays  an  important  part  in  the  collection 
of  sound  waves.  In  man,  in  whom  the  capability  of  moving  the  pinna 
has  been  lost,  it  is  doubtful  if  it  is  at  all  necessary  for  hearing.  Never- 
theless an  individual  with  dull  hearing  may  have  the  perception  of  sound 
increased  by  placing  the  pinna  at  an  angle  of  45  degrees  to  the  side  of  the 
head.  The  external  auditory  canal  transmits  the  sonorous  vibrations  to 
the  tympanic  membrane.  Owing  to  the  obliquity  of  this  canal  it  has 
been  supposed  that  the  waves,  concentrated  at  the  concha,  undergo  a 
series  of  reflections  on  their  way  to  the  tympanic  membrane,  and,  owing 
to  the  position  of  this  membrane,  strike  it  almost  perpendicularly. 

Function  of  the  Tympanic  Membrane. — The  function  of  the  tympanic 
membrane  appears  to  be  in  the  reception  of  sound  vibrations  by  being 
16 


242  HUMAN  PHYSIOLOGY 

thrown  by  them  into  reciprocal  vibrations  which  correspond  in  intensity 
and  amplitude.  That  this  membrane  actually  reproduces  all  vibrations 
within  the  range  of  audibility  has  been  experimentally  demonstrated. 
The  membrane  not  being  fixed,  so  far  as  its  tension  is  concerned,  does  not 
possess  a  fixed  fundamental  note,  like  a  stationary  fixed  membrane,  and  is, 
therefore,  just  as  well  adapted  for  the  reception  of  one  set  of  vibrations  as 
for  another.  This  is  made  possible  by  variations  in  its  tension  in  ac- 
cordance with  the  pitch  of  the  sounds.  In  the  absence  of  all  sound  the 
membrane  is  in  a  condition  of  relaxation;  with  the  advent  of  sound  waves 
possessing  a  gradual  increase  of  pitch,  as  in  the  ascent  of  the  music  scale, 
the  tension  of  the  tympanic  membrane  is  gradually  increased  until  its 
maximum  tension  is  reached  at  the  upper  limit  of  the  range  of  audibility. 
By  this  change  in  tension  certain  tones  become  perceptible  and  distinct, 
while  others  become  indistinct  and  inaudible. 

Function  of  the  Tensor  Tympani  Muscle. — The  function  of  this  muscle 
is,  as  its  name  indicates,  to  increase  the  tension  of  the  membrane  in  ac- 
cordance with  the  pitch  of  the  sound  wave.  The  tension  of  this  muscle 
playing  over  the  processus  cochleariformis  and  attached  at  also  a  right 
angle  to  the  handle  of  the  malleus  will,  when  the  muscle  contracts,  pull 
the  handle  inward,  increase  the  convexity  of  the  membrane,  and  at  the 
same  time  increase  its  tension;  with  the  relaxation  of  this  muscle,  the 
handle  of  the  malleus  passes  outward  and  the  tension  is  diminished.  The 
contractions  of  the  tensor  muscle  are  reflex  in  character  and  excited  by 
nerve  impulses  reaching  it  through  the  small  petrosal  nerve  and  otic 
ganglion.  The  number  of  nerve  stimuli  passing  to  the  muscle  and  deter- 
mining the  degree  of  contraction  will  depend  upon  the  pitch  of  the  sound 
wave  and  the  subsequent  excitation  of  the  auditory  nerve.  The  tensor 
tympani  muscle  may  be  regarded  as  an  accommodative  apparatus  by  which 
the  tympanic  membrane  is  so  adjusted  as  to  enable  it  to  receive  vibra- 
tions of  varying  degrees  of  pitch. 

Function  of  the  Ear  Bones. — The  function  of  the  chain  of  bones  is  to 
transmit  the  sound  wave  across  the  tympanic  cavity  to  the  internal  ear. 
The  first  of  these  bones,  the  malleus,  being  attached  to  the  tympanic 
membrane,  will  take  up  the  vibrations  much  more  readily  than  if  no  mem- 
brane intervened.  Owing  to  the  character  of  the  articulations  when  the 
handle  of  the  malleus  is  drawn  inward,  the  position  of  the  bones  is  so 
changed  that  they  form  practically  a  solid  rod,  and  are  therefore  much 
better  adapted  for  the  transmission  of  molecular  vibrations  than  if  the 
articulations  remained  loose.  As  the  stapes  bone  is  somewhat  shorter 
than  the  malleus,  its  vibrations  are  slighter  than  those  of  the  tympanic 


SENSE    OF  HEARING  243 

membrane,  and  by  this  arrangement  the  amplitude  of  the  vibrations  is 
diminished,  but  their  force  increased. 

The  function  of  the  stapedius  muscle  is,  according  to  Henle,  to  fix 
the  stapes  bone  so  as  to  prevent  too  great  a  movement  from  being  com- 
municated to  it  from  the  incus  and  transmitted  to  the  perilymph.  It 
may  be  looked  upon,  therefore,  as  a  protective  muscle. 

The  Function  of  the  Eustachian  Tube. — The  function  of  the  Eustachian 
tube  is  to  maintain  a  free  communication  between  the  cavity  of  the  middle 
ear  and  the  nasopharynx.  The  pressure  of  air  within  and  without  the 
ear  is  thus  equalized,  and  the  vibrations  of  the  tympanic  membrane  are 
permitted  to  attain  their  maximum,  one  of  the  conditions  essential 
for  the  reception  of  sound  waves.  The  impairment  in  the  acuteness  of 
hearing  which  is  caused  by  an  unequal  pressure  of  the  air  in  the  middle 
ear  can  be  shown — 

1.  By  closing  the  mouth  and  nose  and  forcing  air  from  the  lungs  through 
the  Eustachian  tube  into  the  ear,  producing  an  increase  in  pressure. 

2.  By  closing  the  nose  and  mouth,  and  making  efforts  at  deglutition 
which  withdraws  the  air  from  the  ear  and  diminishes  its  pressure. 

In  both  instances  the  free  vibrations  of  the  tympanic  membrane  are 
interfered  with.  The  pharyngeal  orifice  of  the  Eustachian  tube  is  opened 
by  the  action  of  certain  of  the  muscles  of  deglutition — viz.,  the  levator 
palati,  the  tensor  palati,  and  the  palato-pharyngei  muscles. 

The  Internal  Ear. — The  internal  ear,  or  labyrinth,  is  located  in  the 
petrous  portion  of  the  temporal  bone,  and  consists  of  an  osseous  and  a 
membrane  portion. 

The  osseous  labyrinth  is  divisible  into  three  parts — viz.,  the  vestibule, 
the  semicircular  canals,  and  the  cochlea. 

The  Vestibule  is  a  small  tringular-shaped  cavity  between  the  semi- 
circular canals  and  the  cochlea.  It  is  separated  from  the  cavity  of  the 
middle  ear  by  an  osseous  partition  which  presents  near  its  center  an 
oval  opening,  the  foramen  ovale.  In  the  living  condition  this  opening 
is  closed  by  the  base  of  the  stapes  bone,  which  is  held  in  position  by  an 
annular  ligament.  The  inner  wall  presents  a  number  of  openings  for 
the  passage  of  nerve-fibers. 

The  Semi-circular  canals  are  three  in  number,  and  named  from  their 
position,  the  superior  vertical,  the  posterior  vertical  and  the  horizontal. 
These  canals  are  at  right  angles  one  to  the  other  and  open  by  five  orifices 
into  the  vestibule,  one  of  the  orifices,  however,  being  common  to  two 
of  the  canals.     Each  canal  near  the  vestibular  orifice  is  enlarged  to  al- 


244  HUMAN   PHYSIOLOGY 

most  twice  the  size  of  the  rest  of  the  canal,  forming  what  has  been  termed 
the  ampulla. 

The  Cochlea  forms  the  anterior  part  of  the  internal  ear.  It  is  a  gradually 
tapering  canal,  about  i  J^  inches  in  length,  which  winds  spirally  around  a 
central  axis,  the  modiolus,  two  and  one  half  times.  The  interior  of  the 
cochlea  is  partly  divided  into  two  passages  by  a  thin  plate  of  bone,  the 
lamina  osseous  spiralis,  which  projects  from  the  central  axis  two  thirds 
of  the  way  across  the  canal.  These  passages  are  termed  the  scala  ves- 
tihuli  and  the  scala  tympani,  from  their  communication  with  the  vestibule 
and  tympanum.  The  scala  tympani  communicates  with  the  middle 
ear  through  the  foramen  rotundum,  which,  in  the  natural  condition,  is 
closed  by  the  second  membrana  tympani;  superiorly  they  are  united 
by  an  opening,  the  helicotrema. 

The  whole  interior  of  the  labyrinth,  the  vestibule,  the  semicircular 
canals,  and  the  scala  of  the  cochlea,  contains  a  clear,  limpid  fluid,  the 
perilymph. 

The  membranous  labyrinth  corresponds  to  the  osseous  labyrinth 
with  respect  to  form,  though  it  is  somewhat  smaller  in  size. 

The  vestibular  portion  consists  of  two  small  sacs,  the  utricle  and  the 
saccule. 

The  semicircular  canals  communicate  with  the  utricle  in  the  same 
manner  as  the  bony  canals  communicate  with  the  vestibule.  The  saccule 
communicates  with  the  membranous  cochlea  by  the  canalis  reuniens. 
In  the  interior  of  the  utricle  and  saccule,  at  the  entrance  of  the  auditory 
nerve,  are  small  masses  of  carbonate  of  lime  crystals,  constituting  the 
otoliths.     Their  function  is  unknown. 

The  membranous  cochlea  is  a  closed  tube,  commencing  by  a  blind 
extremity  at  the  first  turn  of  the  cochlea,  and  terminating  at  its  apex 
by  a  blind  extremity  also.  It  is  situated  between  the  edge  of  the  osseous 
lamina  spiralis  and  the  outer  wall  of  the  bony  cochlea,  and  follows  it 
in  its  turns  around  the  modiolus. 

A  transverse  section  of  the  cochlea  shows  that  it  is  divided  into  two 
portions  by  the  osseous  lamina  and  the  basilar  membrane: 

1.  The  scala  vestibuli,  bounded  by  the  periosteum  and  miembrane  of 
Reissner. 

2.  The  scala  lympania,  occupying  the  inferior  portion,  and  bounded 
above  by  the  septum,  composed  of  the  osseous  lamina  and  the  membrana 
basilaris. 

The  true  membranous  canal  is  situated  between  the  membrane  of 
Reissner  and  the  basilar  membrane.     It  is  triangular  in  shape,  but  is 


SENSE    OF   HEARING  245 

partly  divided  into  a  triangular  portion  and  a  quadrilateral  portion  by  the 
tectorial  membrane. 

The  Organ  of  Corti. — The  organ  of  Corti  is  situated  in  the  quad- 
rilateral portion  of  the  canal,  and  consists  of  pillars  of  rods  of  the  consis- 
tence of  cartilage.  They  are  arranged  in  two  rows — the  one  internal, 
the  other  external;  these  rods  rest  upon  the  basilar  membrane;  their 
bases  are  separated  from  one  another,  but  their  upper  extremities  are 
united,  forming  an  arcade.  In  the  internal  row  it  is  estimated  there  are 
about  3,500  and  in  the  external  row  about  5,200  of  these  rods. 

On  the  inner  side  of  the  internal  row  is  a  single  layer  of  elongated  hair- 
cells;  on  the  outer  surface  of  the  external  row  are  three  such  layers  of 
hair-cells.     Nothing  definite  is  known  as  to  their  function. 

The  endolymph  occupies  the  interior  of  the  utricle,  saccule,  and  mem- 
branous canals,  and  bathes  the  structures  in  the  interior  of  the 
membranous  cochlea  throughout  its  entire  extent. 

The  Auditory  Nerve. — The  auditory  nerve  at  the  bottom  of  the 
internal  auditory  meatus  divides  into — 

1.  A  vestibular  branch,  which  is  distributed  to  the  utricle  and  to  the 
semicircular  canals. 

2.  A  cochlear  branch,  which  passes  into  the  central  axis  at  its  base  and 
ascends  to  its  apex ;  as  it  ascends,  fibers  are  given  off,  which  pass  between 
the  plates  of  the  osseous  lamina,  to  be  ultimately  connected  with  the 
organ  of  Corti. 

The  Function  of  the  Semicircular  Canals. — The  function  of  the  semi- 
circular canals  appears  to  be  to  assist  in  maintaining  the  equilibrium  of 
the  body;  destruction  of  the  vertical  canal  is  followed  by  an  oscillation  of 
the  head  upward  and  downward;  destruction  of  the  horizontal  canal  is 
followed  by  oscillations  from  left  to  right.  When  the  canals  are  injured 
on  both  sides,  the  animal  loses  the  power  of  maintaining  equilibrium  upon 
making  muscular  movements.  From  these  facts  it  is  apparent  that  they 
are  among  the  peripheral  sense-organs,  the  physiologic  action  of  which  is 
the  development  of  nerve  impulses,  which  when  transmitted  to  the  brain 
assist  the  equilibratory  mechanism  to  maintain  the  equilibrium  of  the 
body,  both  in  the  standing  position  and  in  the  various  modes  of  progres- 
sion. The  character  of  the  stimulus,  however,  and  the  manner  in  which 
it  acts  on  the  specialized^ portion  of  the  sense-organs  (the  hair-cells)  is 
not  entirely  clear. 

The  Functions  of  the  Cochlea. — The  cochlea  is  the  portion  of  the 
internal  ear  which  is  concerned  in  the  perception  of  tones.    The  arrange- 


246  HUMAN  PHYSIOLOGY 

ment  of  the  histologic  elements  of  the  organ  of  Corti  indicates  that  they 
in  some  way  respond  to  the  vibrations  of  varying  frequency  and  form,  and 
through  the  development  of  nerve  impulses,  evoke  the  sensations  of  pitch 
and  quality.  The  manner  in  which  this  is  accomplished  is  largely  a 
matter  of  speculation. 

Function  of  the  Cochlea. — It  is  regarded  as  possessing  the  power  of 
appreciating  the  quality  of  pitch  and  the  shades  of  di£ferent  musical  tones. 
The  elements  of  the  organ  of  Corti  are  analogous,  in  some  respects,  to  a 
musical  instrument,  and  are  supposed,  by  Helmholtz,  to  be  turned  so  as 
to  vibrate  in  unison  with  the  different  tones  conveyed  to  the  internal  ear. 

Summary. — The  waves  of  sound  are  gathered  together  by  the  pinna 
and  external  auditory  meatus,  and  conveyed  to  the  membrana  tympani. 
This  membrane,  made  tense  or  lax  by  the  action  of  the  tensor  tympani 
muscle,  is  enabled  to  receive  sound  waves  of  either  high  or  low  pitch. 
The  vibrations  are  conducted  across  the  middle  ear  by  a  chain  of  bones  to 
the  foramen  ovale,  and  by  the  column  of  air  of  the  tympanum  to  the  fora- 
men rotundum,  which  is  closed  by  the  second  membrana  tympani,  the 
pressure  of  the  air  in  the  tympanum  being  regulated  by  the  Eustachian 
tube. 

The  internal  ear  finally  receives  the  vibrations,  which  excite  vibrations 
successively  in  the  perilymph,  the  walls  of  the  membranous  labyrinth,  the 
endolymph,  and,  lastly,  the  terminal  filaments  of  the  auditory  nerve,  by 
which  they  are  conveyed  to  the  brain  and  evoke  in  the  cortical  cells  the 
sensations  of  sound. 

PRONATION— ARTICULATE  SPEECH 

Phonation,  the  emission  of  vocal  sounds,  is  accomplished  by  the  vibra- 
tion of  two  elastic  membranes  which  cross  the  lumen  of  the  larynx  antero- 
posteriorly  and  which  are  thrown  into  vibration  by  a  blast  of  air  from  the 
lungs. 

Articulate  speech  is  a  modification  of  the  vocal  sounds  or  the  voice  pro- 
duced by  the  teeth  and  the  muscles  of  the  lips  and  tongue  and  is  employed 
for  the  expression  of  ideas. 

The  larynx,  the  organ  of  the  voice,  is  situated  in  the  fore  part  of  the 
neck,  occupying  the  space  between  the  hyoid  bone  and  the  upper  extremity 
of  the  trachea.  In  this  situation  it  communicates  with  the  cavity  of  the 
pharynx  above  and  the  cavity  of  the  trachea  below.  From  its  anatomic 
relations  and  its  internal  structure — the  interpolation  of  the  elastic 
membranes — the  larynx  subserves  the  two  widely  different  yet  related 
functions,  respiration  and  phonation. 


PHONATION — ARTICULATE   SPEECH  247 

The  larynx  consists  primarily  of  cartilages,  the  more  important  of 
which  are  the  thyroid,  the  cricoid  and  the  arytenoids,  united  one  to  another 
in  such  a  manner  as  to  form  a  more  or  less  rigid  framework  possessing  in 
its  different  joints  a  certain  amount  of  motion;  secondarily  of  muscles 
and  nerves  which  conjointly  impart  to  the  cartilages  the  degree  of  move- 
ment necessary  to  the  performance  of  the  laryngeal  functions.  The 
larynx  is  lined  throughout  by  mucous  membrane  and  covered  externally 
by  fibrous  tissue. 

The  Vocal  Bands. — The  mucous  membrane,  as  it  passes  downward, 
is  reflected  over  the  superior  thyro-arytenoid  ligament,  and  assists  in 
the  formation  of  the  false  vocal  band;  it  then  passes  into  and  lines  the 
ventricle,  after  which  it  is  reflected  outward  over  the  superior  border  of  the 
thyro-arytenoid  muscle  and  ligament,  and  assists  in  the  formation  of 
the  true  vocal  band;  it  then  returns  upon  itself  and  passes  downward 
over  the  lateral  portion  of  the  crico-thyroid  membrane  into  the  trachea. 

The  thin,  free,  reduplicated  edge  of  the  mucous  membrane  constitutes 
the  true  vocal  band.  The  surface  of  the  mucous  membrane  is  covered  by 
ciliated  epithelium  except  in  the  immediate  neighborhood  of  the  vocal 
bands. 

The  weal  hands  are  attached  anteriorly  to  the  thyroid  cartilage  near 
the  receding  angle  and  posteriorly  to  the  vocal  processes  of  the  arytenoid 
cartilages.  They  vary  in  length  in  the  male  from  20  to  25  mm.  and  in  the 
female  from  15  to  20  mm. 

The  Muscles  of  the  Lamyx. — The  muscles  which  have  a  direct  action 
on  the  cartilages  of  the  lamyx  and  determine  the  position  of  the  vocal 
bands  both  for  respiratory  and  phonatory  purposes,  and  which  regulate 
their  tension  as  well,  are  nine  in  number  and  take  their  names  from  their 
points  of  origin  and  insertion:  viz.,  two  posterior  crico-arytenoidsy  two 
lateral  crico-arytenoids,  two  thyroid-arytenois,  one  arytenoid,  and  two  crico- 
thyroids. 

The  posterior  crico-arytenoid  muscles  rotate  the  arytenoid  cartilages 
outward  and  thus  separate  the  vocal  bands  and  enlarge  the  aperture  of 
the  glottis,  a  condition  necessary  to  the  free  entrance  of  the  air  into  the 
lungs.  Since  the  contraction  of  the  crico-arytenoids  has  this  result  they 
are  frequently  spoken  of  as  the  abductor  or  the  respiratory  muscle. 

The  lateral  crico-arytenoid  muscles  are  the  antagonists  of  the  former. 
Their  action  is  to  rotate  the  arytenoid  cartilages  inward  thus  approximat- 
ing the  vocal  bands. 

The  arytenoid  muscle  consists  (i)  of  transversely  arranged  fibers  which 
arise  from  and  are  inserted  into  the  outer  surface  of  the  opposite  arytenoid 


248  HUMAN  PHYSIOLOGY 

cartilages,  and  (2)  of  obliquely  directed  fibers  which  arise  from  the  outer 
angle  of  one  arytenoid  to  be  inserted  into  the  apex  of  the  other.  In  their 
course  they  decussate  in  the  median  line.  The  action  of  this  muscle  is  to 
approximate  the  arytenoid  cartilages  and  thus  obliterate  that  portion  of 
the  glottis  between  the  vocal  processes,  the  rima  respiratoria. 

The  thyro-arytenoid  muscles,  acting  in  conjunction  with  the  lateral 
crico-arytenoids,  closely  approximate  the  edges  of  the  vocal  bands  so 
that  the  space  between  them  is  reduced  to  a  mere  slit — the  rima  vocalis — 
one  of  the  conditions  necessary  for  phonation. 

Collectively  these  muscles  adduct  the  vocal  bands  to  the  middle  line 
and  thus  constrict  the  glottis.  For  this  reason  they  are  generally  spoken 
of  as  the  adductors  or  the  phonatory  muscles. 

The  crico-thyroid  muscle  at  the  time  of  its  contraction  draws  up  the 
anterior  part  of  the  cricoid  cartilage  toward  the  thyroid,  which  remains 
stationary,  and  swings  the  quadrate  plate  of  the  cricoid  and  the  arytenoid 
cartilages  downward,  and  backward.  This  movement  has  the  result  of 
tensing  the  vocal  bands.  The  cricoid  is  at  the  same  time  drawn  backward 
by  the  action  of  the  more  longitudinally  disposed  fibers. 

Movements  of  the  Vocal  Bands. — During  the  intervals  of  speaking  the 
vocal  bands  are  widely  separated  by  the  tonic  contraction  of  the  poste- 
rior crico-arytenoid  muscles.  With  each  inspiration,  however,  they  are 
separated  to  a  somewhat  greater  extent;  with  each  expiration  they  return 
to  their  former  condition. 

Phonation. — As  soon  as  phonation  is  about  to  take  place  the  vocal 
bands  are  suddenly  approximated,  made  parallel,  and  increased  in  tension. 
When  the  foregoing  conditions  in  the  glottis  are  realized,  the  air  stored 
or  collected  in  the  lungs  is  forced  by  the  contraction  of  the  expiratory 
muscles,  through  the  narrow  space  between  the  bands.  As  a  result  of  the 
resistance  offered  by  this  narrow  outlet  and  the  force  of  the  expiratory 
muscles,  the  air  within  the  lungs  and  trachea  is  subjected  to  pressure,  and 
as  soon  as  the  pressure  attains  a  certain  level  the  vocal  bands  are  thrown 
into  vibrations,  which  in  turn  impart  to  the  column  of  air  in  the  upper 
air-passages  a  corresponding  series  of  vibrations  by  which  the  laryngeal 
vibrations  are  reinforced. 

The  Characteristics  of  the  Vocal  Sounds. — All  vocal  sounds  are  charac- 
terized by  intensity,  pitch  and  quality. 

The  intensity  or  loudness  of  a  second  depends  on  the  extent  or  amplitude 
of  the  to-and-fro  vibration,  or  the  extent  of  the  excursion  of  the  vocal 
band  on  either  side  of  the  position  of  equilibrium  or  rest;  and  this  in  turn 
depends  on  the  force  with  which  the  blast  of  air  strikes  the  band. 


PHONATION — ARTICULATE   SPEECH  249 

The  pitch  of  the  voice  depends  on  the  number  of  vibrations  in  a  unit 
of  time,  a  second.  This  will  be  conditioned  by  the  length  of  the  bands  in 
vibration  or  the  length  and  width  of  the  aperture  through  which  the  air 
passes  and  the  degree  of  tension  to  which  the  bands  are  subjected.  In 
the  emission  of  sounds  of  highest  pitch  the  tension  of  the  vocal  bands 
and  the  narrowing  of  the  glottis  attain  their  maximum.  In  the  emission 
of  sounds  of  lowest  pitch  the  reverse  conditions  obtain.  In  passing  from 
the  lowest  to  the  highest  pitched  sounds  in  the  range  of  the  voice  peculiar 
to  any  one  individual,  there  is  a  progressive  increase  in  both  the  tension 
of  the  vocal  bands  and  the  narrowing  of  the  glottic  aperture. 

The  quality  of  the  voice,  the  timbre  or  tone-color,  depends  on  tht  form 
combined  with  the  intensity  and  pitch  of  the  vibration.  As  with  sounds 
produced  by  musical  instruments,  the  primary  of  fundamental  vibration 
of  the  vocal  band  is  complicated  by  the  superposition  of  secondary  or 
partial  vibrations  (overtones) .  The  form  of  the  vibration  will,  therefore 
be  a  resultant  of  the  blending  of  a  number  of  different  vibrations.  The 
quality  of  the  sound  produced  in  the  larynx  is,  however,  modified  by  the 
resonance  of  the  mouth  and  nasal  cavities;  certain  of  the  overtones  being 
reinforced  by  changes  in  the  shape  of  the  mouth  cavity  more  especially, 
thus  giving  to  the  voice  a  somewhat  different  quality. 

Speech  is  the  expression  of  ideas  by  means  of  articulate  sounds.  These 
sounds  may  be  divided  into  vowel  and  consonant  sounds. 

The  vowel  sounds,  a,  c,  i,  0,  w,  are  laryngeal  tones  modified  by  the 
superposition  and  reinforcement  of  certain  overtones  developed  in  the 
mouth  and  pharynx  by  changes  in  their  shapes.  The  number  of  vibra- 
tions underlying  the  production  of  each  vowel  sound  is  a  matter  of 
dispute. 

Consonant  sounds  are  produced  by  the  more  or  less  complete  interrup- 
tion of  the  vowel  sounds  during  their  passage  through  the  organs  of 
speech.     These  may  be  divided  into: 

1.  Labials,  ^,  6,  ?w. 

2.  Labio-dentals,  /,  ?>. 

3.  Linguo-dentals,  s,  z, 

4.  Anterior  linguo-palatals,  t,  d,  I,  n,  r,  sh^  zh. 

5.  Posterior  linguo-palatals,  k,  g,  /?,  y. 

The  names  of  these  different  groups  of  consonants  indicate  the  region 
of  the  mouth  in  which  they  are  produced  and  the  means  by  which  the  air 
blast  is  interrupted. 


2  so  HUMAN  PHYSIOLOGY 

The  Nerves  of  the  Larynx. — The  two  antagonistic  groups  of  laryngeal 
muscles — the  respiratory  and  the  phonatory — are  innervated  by  two  dif- 
ferent groups  of  nerve-fibers  both  of  which  however  are  contained  in  the 
trunk  of  the  inferior  laryngeal  nerve.  These  two  groups  of  nerve-fibers 
have  their  origin  in  two  separate  centers  in  the  floor  of  the  fourth  ventricle 
of  the  medulla.  These  centers  are  known  as  the  laryngeal  respiratory 
and  the  phonatory  centers.  The  phonatory  center  in  the  medulla  is  in 
relation  with  a  volitional  or  motor  center  in  the  lower  portion  of  the 
precentral  convolution  near  the  anterior  border.  Stimulation  of  this  area 
is  invariably  followed  by  bilateral  adduction  of  the  vocal  bands  and 
closure  of  the  glottis. 


REPRODUCTION 

Reproduction  is  the  function  by  which  the  species  is  preserved;  it  is 
accomplished  by  the  organs  of  generation  in  the  two  sexes.  Embryology 
is  the  science  which  investigates  the  successive  stages  in  the  development 
of  the  embryo. 

GENERATIVE  ORGANS  OF  THE  FEMALE 

The  generative  organs  of  the  female  consist  of  the  ovaries,  Fallopian 
tubes,  uterus,  and  vagina. 

The  ovaries  are  two  small,  flattened  bodies,  measuring  about  40  mm. 
in  length  and  20  mm.  in  width;  they  are  situated  in  the  cavity  of  the 
pelvis,  and  are  imbedded  in  the  posterior  layer  of  the  broad  ligament; 
attached  to  the  uterus  by  a  round  ligament,  and  to  the  extremities  of  the 
Fallopian  tubes  by  the  fimbriae.  The  ovary  consists  of  an  external  mem- 
brane of  fibrous  tissue,  the  cortical  portion,  in  which  are  embedded  the 
Graafian  vesicles^  and  an  internal  portion,  the  stroma^  containing 
blood-vessels. 

The  Graafian  vesicles  are  exceedingly  numerous,  but  are  situated  only 
in  the  cortical  portion.  It  is  estimated  that  each  ovary  contains  from 
20,000  to  40,000  follicles.  Although  the  ovary  contains  the  vesicles  from 
the  period  of  birth,  it  is  only  at  puberty  that  they  attain  their  full  develop- 
ment. From  this  time  onward  to  the  catamenial  period  there  is  a  constant 
growth  and  maturation  of  the  Graafian  vesicles.  They  consist  of  an 
external  investment,  composed  of  fibrous  tissues  and  blood-vessels,  in 
the  interior  of  which  is  a  layer  of  cells  forming  the  membrana  granulosa;, 
at  its  lower  portion  there  is  an  accumulation  of  cells,  the  proligerous  disc; 
in  which  the  ovum  is  contained.  The  cavity  of  the  vesicle  contains  a 
slightly  yellowish  alkaline,  albuminous  fluid. 

The  ovum  is  a  globular  body,  measuring  about  0.3  mm.  in  diameter. 
It  consists  of  a  mass  of  protoplasmic  material  cytoplasm,  a  nucleus  or" 
germinal  vesicle  and  a  nucleolus  or  germinal  spot.  The  peripheral  portion 
of  the  cytoplasm  is  surrounded  by  a  clear  thick  membrane,  the  zona 
pellucida,  external  to  which  is  a  layer  of  radially  placed  columnar  epithe- 
lium forming  the  corona  radiata.  The  nucleus  consists  of  a  nuclear 
membrane  enclosing  material,  some  of  which  arranged  in  the  form  of 
thread  stains  readily  and  hence  known  as  chromatin,  in  the  meshes  of 
which  lies  a  material  that  stains  faintly  and  hence  known  as  achromatin. 

The  Fallopian  tubes  are  about  12  centimeters  in  length,  and  extend 
outward  from  the  upper  angles  of  the  uterus,  between  the  folds  of  the 

251 


252  HUMAN   PHYSIOLOGY 

broad  ligaments,  and  terminate  in  a  fringed  extremity  which  is  attached 
by  one  of  the  fringes  to  the  ovary.     They  consist  of  three  coats: 

1.  The  external,  or  peritoneal. 

2.  Middle,  or  muscular,  the  fibers  of  which  are  arranged  in  a  circular 
and  longitudinal  direction. 

3.  Internal,  or  mucous,  usually  folded  longitudinally,  is  covered  with 
ciliated  epithelial  cells,  which  are  always  waving  from  the  ovary  toward 
the  uterus. 

The  uterus  is  pyriform  in  shape,  and  may  be  divided  into  a  body  and 
neck;  it  measures  about  7  cm.  in  length  and  5  cm.  in  breadth  in  the 
unimpregnated  state.  At  the  lower  extremity  of  the  neck  is  the  os 
externum;  at  the  junction  of  the  neck  with  the  body  is  a  constriction, 
the  OS  internum.  The  cavity  of  the  uterus  is  triangular  in  shape,  the 
walls  of  the  triangle  being  almost  in  contact. 

The  walls  of  the  uterus  are  made  up  of  many  layers  of  non-striated 
muscle-fibers,  covered  externally  by  peritoneum,  and  lined  internally  by 
mucous  membrane,  containing  numerous  tubular  glands,  and  covered  by 
ciliated  epithelial  cells. 

The  vagina  is  a  membranous  canal,  from  12  to  18  cm.  in  length,  situated 
between  the  rectum  and  bladder.  It  extends  obliquely  upward  from 
the  surface,  almost  to  the  brim  of  the  pelvis,  and  embraces  at  its  upper 
extremity  the  neck  of  the  uterus. 

Discharge  of  the  Ovum. — As  the  Graafian  vesicle  matures  it  increases 
in  size,  from  an  augmentation  of  its  liquid  contents,  and  approaches  the 
surface  of  the  ovary,  where  it  forms  a  projection,  measuring  from  six  to 
twelve  cm.  The  maturation  of  the  vesicle  occurs  periodically,  about 
every  twenty-eight  days,  and  is  attended  by  the  phenonena  of  menstrua- 
tion. During  this  period  of  active  congestion  of  the  reproductive  organs 
the  Graafian  vesicle  ruptures,  the  ovum  and  liquid  contents  escape,  and 
are  caught  by  the  fimbriated  extremity  of  the  Fallopian  tube,  which  has 
adapted  itself  to  the  posterior  surface  of  the  ovary.  The  passage  of  the 
ovum  through  the  Fallopian  tube  into  the  uterus  occupies  from  ten  to 
fourteen  days,  and  is  accomplished  by  muscular  contraction  and  by  the 
action  of  the  ciliated  epithelium. 

Menstruation  is  a  periodic  discharge  of  blood  from  the  mucous  mem- 
brane of  the  uterus,  due  to  a  fatty  degeneration  of  the  small  blood-vessels. 
Under  the  pressure  of  an  increased  amount  of  blood  in  the  reproductive 
organs,  attending  the  process  of  ovulation,  the  blood-vessels  rupture,  and 
a  hemorrhage  takes  place  into  the  uterine  cavity;  thence  it  passes  into 


REPRODUCTION 


253 


the  vagina/    Menstruation  lasts  from  five  to  six  days,  and  the  amount 
of  blood  discharged  averages  from  180  to  200  c.c. 

Corpus  Luteum. —  For  some  time  previous  to  the  rupture  of  a  Graafian 
vesicle  it  increases  in  size  and  becomes  vascular;  its  walls  become  thickened 
from  the  deposition  of  a  reddish-yellow,  glutinous  substance,  a  product 
of  cell  growth  from  the  proper  coat  of  the  follicle  and  the  membrana 
granulosa.  After  the  ovum  escapes  there  is  usually  a  small  effusion  of 
blood  into  the  cavity  of  the  follicle,  which  soon  coagulates,  loses  its  color- 
ing-matter, and  acquires  the  characteristics  of  fibrin,  but  it  takes  no  part 
in  the  formation  of  the  corpus  luteum.  The  walls  of  the  follicle  become 
convoluted  and  vascular  and  undergo  hypertrophy,  until  they  occupy  the 
whole  of  the  follicular  cavity.  At  its  period  of  fullest  development  the 
corpus  luteum  measures  20  mm.  and  12  mm.  in  depth.  In  a  few  weeks 
the  mass  loses  its  red  color  and  becomes  yellow,  constituting  the  corpus 
luteum  J  or  yellow  body.  It  then  begins  to  retract  and  becomes  pale;  and 
at  the  end  of  two  months  nothing  remains  but  a  small  cicatrix  upon  the 
surface  of  the  ovary.  Such  are  the  changes  in  the  follicle  if  the  ovum 
has  not  been  impregnated. 

The  corpus  luteum,  after  impregnation  has  taken  place,  undergoes  a 
much  slower  development,  becomes  larger,  and  continues  during  the  entire 
period  of  gestation.  The  difference  between  the  corpus  luteum  of  the 
unimpregnated  and  pregnant  condition  is  expressed  in  the  following  table 
by  Dalton: 


Corpus  Luteum  of  Menstruation.    Corpus  Luteum  of  Pregnancy 


At  the  end  of  three  I     20  mm 
weeks.  pale. 

One  month Smaller;  convoluted 

I  wall    bright    yellow; 
!  clot  still  reddish. 
Two  months Reduced  to  the  con- 
dition of  an  insignifi- 
cant cicatrix. 

Four  months Absent     or     unno- 

ticeable. 


Six  months 


Nine  months. 


Absent. 


Absent . 


in  diameter;  central  clot  reddish;  convoluted  wall 

Larger;     convoluted     wall     bright 
yellow;  clot  still  reddish. 


20  mm.  in  diameter;  convoluted 
wall  bright  yellow;  clot  perfectly 
decolorized. 

20  mm.  in  diameter;  clot  pale 
and  fibrinous;  convoluted  wall  dull 
yellow. 

Still  as  large  as  at  the  end  of 
second  month;  clot  fibrinous; 
convoluted  wall  paler. 

12  mm.  in  diameter;  central  clot 
converted  into  a  radiating  cicatrix; 
external  wall  tolerably  thick  and 
convoluted,  but  without  any  bright 
yellow  color. 


254  HUMAN  PHYSIOLOGY 

GENERATIVE  ORGANS  OF  THE  MALE 

The  generative  organs  of  the  male  consists  of  the  testicles,  vasa  defer- 
entia,  vesiculae  seminales,  and  penis. 

The  testicles,  the  essential  organs  of  reproduction  in  the  male,  are  two 
oblong  glands,  about  40  mm.  in  length,  compressed  from  side  to  side 
and  situated  in  the  cavity  of  the  scrotum. 

The  proper  coat  of  the  testicles,  the  tunica  albuginea,  is  a  white,  fibrous 
structure,  about  one  mm.  in  thickness;  after  enveloping  the  testicle,  it  is 
reflected  into  its  interior  at  the  posterior  border,  and  forms  a  vertical  pro- 
cess, the  mediastifyum  testis^  from  which  septa  are  given  off,  dividing  the 
testicle  into  lobules. 

The  substance  of  the  testicle  is  made  up  of  the  seminiferous  tubules. 
which  exist  to  the  number  of  840;  they  are  exceedingly  convoluted,  and 
when  unravelled  are  about  30  cm.  in  length.  As  they  pass  toward  the 
apices  of  the  lobules,  they  become  less  convoluted,  and  terminate  in 
from  twenty  to  thirty  straight  ducts,  the  vasca  recta,  which  pass  upward 
through  the  mediastinum  and  constitute  the  rete  testis.  At  the  upper  part 
of  the  mediastinum  the  lobules  unite  to  form  from  nine  to  thirty  small 
ducts,  the  vasa  efferentia,  which  become  convoluted  and  from  the  globus 
major  of  the  epididymis;  the  continuation  of  the  tubes  downward  behind 
the  testicle  and  a  second  convolution  constitutes  the  body  and  globus 
minor. 

The  seminal  tubule  consists  of  a  basement  membrane  lined  by  granular 
nucleated  epithelium. 

The  vas  deferens,  the  excretory  duct  of  the  testicle,  is  about  60  cm. 
in  length,  and  may  be  traced  upward  from  the  epididymis  to  the  under 
surface  of  the  base  of  the  bladder,  where  it  unites  with  the  duct  of  the 
vesicula  seminalis  to  form  the  ejaculatory  duct. 

The  vesiculae  seminales  are  two  lobulated,  pyriform  bodies  about  50 
mm,  in  length,  situated  on  the  inner  surface  of  the  bladder. 

They  have  an  external  fibrous  coat,  a  middle  muscular  coat,  and  an 
internal  mucous  coat,  covered  by  epithelium,  which  secretes  a  mucous 
fluid.  The  vesiculae  seminales  serve  as  reservoirs,  in  which  the  seminal 
fluid  is  temporarily  stored  up. 

The  ejaculatory  duct,  about  20  mm.  in  length,  opens  into  the  urethra, 
and  is  formed  by  the  union  of  the  vasa  deferentia  and  the  ducts  of  the 
vesiculae  seminales. 

The  prostate  gland  surrounds  the  posterior  extremity  of  the  urethra, 
and  opens  into  it  by  from  twenty  to  thirty  openings,  the  orifices  of  the 


REPRODUCTION  255 

prostatic  tubules.     The  gland  secretes  a  fluid  which  forms  part  of  the  semen 
and  assists  in  maintaining  the  vitality  of  the  spermatozoa. 

The  semen  is  a  complex  fluid,  made  up  of  the  secretions  from  the 
testicles,  the  vesiculae  seminales,  the  prostatic  and  urethral  glands.  It 
is  grayish-white  in  color,  mucilaginous  in  consistence,  of  a  characteristic 
odor,  and  somewhat  heavier  than  water.  From  one  to  five  c.c.  is 
ejaculated  at  each  orgasm. 

The  spermatozoa  are  peculiar  anatomic  elements,  developed  within  the 
seminal  tubules,  and  possess  the  power  of  spontaneous  movement.  The 
spermatozoa  consist  of  a  conoid  head  and  a  long,  filamentous  tail,  which 
is  in  continuous  and  active  motion;  so  long  as  they  remain  in  the  vas 
deferens  they  are  quiescent,  but  when  free  to  move  in  the  fluid  of  the 
vesiculae  seminales,  they  become  very  active. 

Origin. — The  spermatozoa  appear  at  the  age  of  puberty,  and  are  then 
constantly  formed  until  an  advanced  age.  They  are  developed  from  the 
nuclei  of  large,  round  cells  contained  in  the  anterior  of  the  seminal  tubules, 
as  many  as  fifteen  to  twenty  developing  in  a  single  cell. 

When  the  spermatozoa  are  introduced  into  the  vagina,  they  pass  readily 
into  the  uterus  and  through  the  Fallopian  tubes  toward  the  ovaries, 
where  they  remain  and  retain  their  vitality  for  a  period  of  from  eight  to 
ten  days. 

Fecundation  is  the  union  of  the  spermatozoa  with  the  ovum  during  its 
passage  toward  the  uterus  and  usually  takes  place  in  the  Fallopian  tube 
just  outside  the  uterus.  After  floating  around  the  ovum  in  an  active 
manner,  a  single  spermatozoan  penetrates  the  ovum,  this  accomplished . 
the  head  and  body  meet  and  unite  with  the  nucleus  of  the  ovum.  A  series 
of  histologic  changes  now  arise  which  eventuate  in  the  production  of  a 
new  cell,  the  parent  cell,  from  which  the  new  being  developes  through 
successive  division,  multiplication  and  differentiation  of  cells. 

The  Fixation  of  the  Ovum. — The  ovum  after  fertilization  in  the  oviduct, 
continues  to  divide  and  pass  slowly  to  the  uterus  (8-10  days)  where  it  is 
retained  until  the  end  of  gestation.  A  menstrual  mucosa  having  de- 
veloped, the  ovum  lodges  on  a  smooth  thick  area  and  gradually  sinks 
beneath  the  surface.  During  the  passage  down  the  oviduct  the  zona 
pellucida  has  become  attenuated  and  has  been  finally  replaced  by  a  thick 
layer  of  ameboid  and  phagocytic  cells  called  the  trophoderm.  Upon 
lodgement  of  the  ovum  these  cells  destroy  the  underlying  mucosa  and  pro- 
duce a  cavity  into  which  the  ovum  sinks.  As  the  ovum  increases  in 
size  the  mucosa  gradually  covers  it;  that  portion  of  the  mucosa  toward 


256  HUMAN  PHYSIOLOGY 

the  uterine  cavity  is  called  the  decidua  capsular  is  or  reflexa,  that  beneath 
the  ovum  the  decidua  basilaris  or  placentalis^  while  the  remainder  consti- 
tutes the  decidua  parietalis  or  vera.  As  development  proceeds  the  decidua 
basilaris  becomes  greater  and  ultimately  develops  into  the  placenta,  the 
organ  of  nutrition  and  respiration. 

Segmentaticm  of  the  Ovum. — Immediately  after  fertilization  the  ovum 
divides  and  redivides  within  the  diminishing  zone  pellucida,  forming  an 
irregular  mass  of  cells  called  the  morula.  The  peripheral  cells  form  a  layer, 
the  trophoderm,  beneath  the  attenuated  zona  pellucida,  ultimately  replac- 
ing that  structure.  The  remaining  cells  of  the  morula  differentiate  into 
three  masses,  ectodermal,  entodermal  and  mesodermal.  The  central  cells  of 
these  masses  liquefy  and  disappear  forming  thus  the  ectodermal  or  amniotic 
cavity,  limited  by  the  ectoderm;  the  entodermal  cavity  limited  by  the 
entoderm;  and  the  mesodermal  or  celomic  cavity  limited  by  the  extra- 
embryonic mesoderm.  Meanwhile  cells  in  various  parts  of  the  thickened 
trophoderm  have  disappeared,  leaving  this  layer  in  the  form  of  delicate 
trophodermal  villi,  the  future  chorionic  and  placental  villi. 

The  Embryonic  Shield. — The  floor  of  the  amniotic  cavity  consisting  of 
ectoderm  and  entoderm  constitute  the  embryonic  shield  or  disk.  As  the 
shield  increases  in  size,  a  median  longitudinal  thickening  is  seen  occupy- 
ing the  caudal  half  of  the  area.  This  is  the  primitive  streak,  a  temporary 
structure  that  is  soon  overshadowed  by  changes  in  the  area  just  in  front 
of  it.  Here  is  formed  a  median  longitudinal,  grooved  ridge  of  ectoderm 
that  develops  rapidly  in  length.  This  is  the  neural  groove  and  folds. 
The  dorsal  lips  of  the  groove  approach  each  other  in  the  mid-line  and  fuse, 
separating  from  the  original  ectoderm  which  closes  over  the  ectodermal 
tube.  This  tube  is  the  neural  tube  from  which  the  nerve  system  is  de- 
veloped. In  the  immediate  vicinity  of  the  head  end  of  the  primitive 
streak  is  seen  a  darkened  area,  Hensen^s  node,  that  represents  the  begin- 
ning invagination  of  the  ectoderm  in  the  formation  of  the  embryonic 
mesoderm  and  notochord  to  be  considered  later.  That  portion  of  the 
embryonic  shield  that  gives  rise  to  the  embryo  itself  becomes  distinctly 
outlined  laterally  and  in  the  head  and  tail  regions  of  the  neural  groove. 
Just  external  to  this  area,  the  embryonic  area  proper,  is  a  transparent  area, 
the  area  pellucida,  beyond  which  is  the  area  opaca  in  which  the  first 
blood-vessels  appear. 

Mesoderm  and  Notochord.^ — So  far  in  the  embryonic  area  only  ecto- 
derm and  entoderm  exist.  Hensen's  node,  at  the  head  end  of  the  primi- 
tive streak,  represents  an  invagination  (gastrulation)  of  ectoderm  between 
ectoderm  and  entoderm.     This  invagination  elongates  headward  in  the 


REPRODUCTION  257 

embryonic  area  constituting  a  tube  of  ectodermal  cells,  the  chordal  canal. 
Later  the  ventral  wall  of  the  canal  and  the  adjacent  entoderm  disappear, 
so  that  the  chordal  ectoderm  temporarily  forms  the  dorsal  median  bound- 
ary of  the  entodermal  cavity.  By  this  process  a  communication  is 
established  between  the  entodermal  cavity  and  neural  groove,  called 
the  n euro-enteric  canal.  The  chordal  ectoderm  separates  from  the  ento- 
derm and  then  forms  a  solid  cord  of  cells,  the  notochord;  between  entoderm 
and  neural  groove  the  neurenteric  canal,  however,  persisting  for  some 
time.  In  the  meanwhile,  other  ectodermic  cells  in  the  region  of  the 
chordal  invagination  spread  between  ectoderm  and  entoderm  and  form 
the  anlage  of  the  mesoderm.  These  cells  by  rapid  proliferation  soon 
separate  ectoderm  and  entoderm  and  joins  the  extra-embryonic  mesoderm. 
The  separation  of  these  two  structures  is  complete  except  in  the  regions 
of  the  bucco-pharyngeal  and  cloacal  membranes. 

On  each  side  of  the  neural  groove  the  mesoderm  becomes  transversely 
grooved  in  its  ectodermal  surface  forming  a  number  of  successive  block- 
like masses  called  primitive  somites  or  segments;  of  these,  there  are  thirty- 
eight  for  the  trunk  and  possibly  four  for  the  head  regions.  Each  segment 
consists  of  three  parts,  the  sclerotome^  the  myotome  and  the  dermatome. 
Lateral  to  the  somite  is  a  thickened  mass  of  mesoderm,  the  intermediate- 
cell  masSy  that  laterally  divides  into  two  layers;  the  outer  accompanies  the 
ectoderm  forming  the  somatopleure,  which  gives  rise  to  the  body  wall; 
the  inner  joins  the  entoderm,  forming  the  splanchnopleure  from  which 
the  gut  tract,  vitelline  duct  and  yolksac  are  derived. 

Fetal  Membranes. — As  the  primitive  streak  and  neural  groove  are 
forming,  the  extra-embryonic  mesoderm  that  lies  beneath  the  tropho- 
derm  invades  the  trophodermic  villi,  forming  there  the  chorion  with  its 
villi.  Gradually  the  mesoderm  of  the  roof  of  the  amniotic  cavity  divides 
into  two  layers,  the  upper  constituting  chorionic  mesoderm,  while  the 
under  one  is  attached  to  the  ectoderm  of  the  amniotic,  and  forms  with  the 
latter,  the  Amnion.  In  the  chick  and  some  mammals  the  amnion  is 
derived  from  the  somatopleure  in  the  folding  off  of  the  body.  In  amniotes 
the  amniotic  cavity  is  at  first  small,  but  rapidly  increases  in  size.  It 
contains  a  clear  fluid,  the  amniotic  fluid,  which  amounts  at  term  to  about 
one  quart.  It  serves  to  protect  the  fetus  during  gestation,  and  at  par- 
turition it  dilates  the  os  cervis  and  flushes  the  birth  canal.  This  liquid 
is  derived  mainly  from  the  blood  as  it  contains  albumin,  sugar,  fat  and 
inorganic  salts.  Traces  of  urea  indicate  that  some  of  its  constituents  are 
derived  from  the  embryo  itself. 

The  caudal  end  of  the  embryonic  area  is  left  connected  with  the  chorion 
by  a  heavy  band  of  mesoderm  termed  the  belly-stalk  to  which  the  caudal 
17 


258  HUMAN  PHYSIOLOGY 

part  of  the  amnion  is  attached.  The  entoderm  is  invaginated  into  the 
belly-stalk  for  a  short  distance  constituting  the  allantois  of  higher  forms; 
the  allantois  grows  out  between  the  closing  somatopleure  folds  forming 
the  body  wall  and  constitutes  a  free  sac  upon  which  vessels,  allantoic 
arteries  and  veins ^  develop  from  the  embryo.  This  sac  then  spreads 
beneath  the  white  shell  membrane  forming  the  organ  for  nutrition  and 
respiration  of  these  forms  during  the  last  half  of  their  incubation  periods. 
In  mammals  the  extra-embryonic  portion  of  the  allantois  is  of  little 
importance. 

The  Formation  of  the  Placenta. — The  chorionic  villi  increase  rapidly 
in  size  and  number  and  usually  surround  the  whole  fetal  sac  giving  it  a 
peculiar  shaggy  appearance.  Blood-vessels  now  proceed  from  the  embryo 
along  the  belly-stalk  (not  the  allantois  in  higher  forms  as  formerly  stated). 
There  the  umbilical  arteries  and  veins  pass  to  the  chorionic  villi  and  send 
branches  of  those  of  the  placental  area;  these  vascularized  villi  constitute 
the  chorion  frondosuMy  while  the  avascular  villi  form  the  chorion  leva.  The 
villi  of  the  latter  disappear  during  the  second  month,  leaving  the  chorionic 
membrane  smooth.  The  villi  of  the  chorion  frondosum  now  penetrate  the 
uterine  glands  of  the  decidua  basilaris  which  by  this  time  have  been 
denuded  of  epithelium  and  have  gained  connection  with  the  blood-vessels 
of  the  mucosa;  in  this  manner  these  uterine  glands  have  become  converted 
into  blood  sinuses.  The  chorionic  villi  either  attach  themselves  to  the 
tunica  propria  of  the  mucosa  (fixed  villi)  or  remain  free,  floating  villi.  At 
the  edge  of  the  placental  area  very  few  villi  develop  leaving  a  circular 
channel  called  the  marginal  sinus.  This  attachment  of  the  villi  becomes 
marked  from  the  third  month  on  and  is  considered  the  beginning  of  placen- 
tation.  From  this  time  on  to  full  term  there  is  merely  an  increase  in 
number  of  villi  and  vessels  and  thus  an  increase  in  the  size  of  the  placenta. 

The  placenta  is  the  most  important  of  the  fetal  structures.  As  it 
develops,  conditions  are  established  which  permit  of  a  free  exchange  of 
material  between  mother  and  child.  Whether  by  osmosis  or  by  an  act  of 
secretion,  the  nutritive  materials  of  the  maternal  blood  pass  through  the 
intervening  membrane  into  the  fetal  blood  on  the  one  hand,  while  waste 
products  pass  in  the  reverse  direction  into  the  maternal  blood  on  the  other 
hand.  Inasmuch  as  oxygen  is  absorbed  and  carbon  dioxid  exhaled  by  the 
same  structures,  the  placenta  is  to  be  regarded  as  both  a  digestive  and  a 
respiratory  organ.  So  long  as  these  exchanges  are  permitted  to  take 
place  in  a  normal  manner  the  nutrition  of  the  embryo  is  secured. 

The  Nutrition  of  the  Embryo. — As  the  ovum  passes  down  the  oviduct 
it  imbibes  nutritive  materials  from  the  mucosa.     As  it  lodges  in  the  uterus 


REPRODUCTION  259 

it  is  nourished  at  first  in  the  same  way.  The  first  circulation  developed  is 
the  vitelline,  but  as  the  amount  of  nutritive  material  is  very  small  in  mam- 
mals its  activity  is  limited.  In  the  oviparous  forms,  however,  where  the 
nutritive  material  is  large  in  amount  this  circulation  is  important.  The 
allantoic  circulation  is  likewise  of  importance  in  the  oviparous  forms  and 
constitutes  their  last  fetal  circulation.  In  mammals  the  allantoic  circula- 
tion is  merely  a  transitonal  stage  in  the  formation  of  the  placental 
circulation. 

Circulation  of  Blood  in  the  Fetus. — The  blood  returning  from  the  pla- 
centa, after  having  received  oxygen  and  being  freed  from  carbonic  acid, 
is  carried  by  the  umbilical  vein  to  the  under  surface  of  the  liver;  here  a  por- 
tion of  it,  about  one-half,  passes  through  the  ductus  venosus  into  the  ascend- 
ing vena  cava,  while  the  remainder  flows  through  the  liver  and  passes  into 
the  inferior  vena  cava  by  the  hepatic  veins.  When  the  blood  is  emptied 
into  the  right  auricle,  it  is  directed  by  the  Eustachian  valve  through  the 
foramen  ovale,  into  the  left  auricle,  thence  into  the  left  ventricle,  and  so 
into  the  aorta  and  to  all  parts  of  the  system.  The  venous  blood  returning 
from  the  head  and  upper  extremities  is  emptied,  by  the  superior  vena  cava, 
into  the  right  auricle,  from  which  it  passes  into  the  right  ventricle,  and 
thence  into  the  pulmonary  artery.  Owing  to  the  condition  of  the  lung 
only  a  small  portion  flows  through  the  pulmonary  capillaries,  the  greater 
part  passing  through  the  ductus  arteriosus,  which  opens  into  the  aorta  at 
a  point  below  the  origin  of  the  carotid  and  subclavian  arteries.  The 
mixed  blood  now  passes  down  the  aorta  to  supply  the  lower  extremities, 
but  a  portion  of  it  is  directed,  by  the  hypogastric  arteries,  to  the  placenta, 
to  be  again  oxygenated. 

At  birth,  the  placental  circulation  gives  way  to  the  circulation  of  the 
adult.  As  soon  as  the  child  begins  to  breathe,  the  lungs  expand,  blood 
flows  freely  through  the  pulmonary  capillaries,  and  the  ductus  arteriosus 
begins  to  contract.  The  foramen  ovale  closes  about  the  tenth  day.  The 
umbilical  vein,  the  ductus  venosus,  and  the  hypogastric  arteries  become 
impervious  in  several  days  as  far  as  the  bladder.  Their  distal  ends  ulti- 
mately form  rounded  cords. 

Physiologic  Activities  of  the  Embryo. — During  intrauterine  life  the 
evolution  of  structure  is  accompanied  by  an  evolution  of  function.  The 
relatively  simple  and  uniform  metabolism  of  the  undifferentiated  blasto- 
dermic membranes  gradually  increases  in  complexity  and  variety,  as  the 
individual  tissues  and  organs  make  their  appearance  and  assume  even  a 
slight  degree  of  functional  activity.  As  to  the  periods  at  which  different 
organs  begin  to  functionate,  but  little  is  positively  known. 


26o  HUMAN  PHYSIOLOGY 

The  primitive  heart,  in  all  probability,  begins  to  pulsate  very^^arly,  as 
in  an  embryo  from  fifteen  to  eighteen  days  old  and  measuring  but  2.2  mm. 
in  length,  Coste  found  the  amnion,  the  allantois,  the  omphalo-mesenteric 
vessels,  and  the  two  primitive  aortae  developed.  In  the  earlier  weeks, 
all  products  of  metabolism  are  doubtless  eliminated  by  the  placenta, 
structures;  but  as  metabolism  increases  in  complexity  the  liver  and  kidney 
assume  excretory  activity.  Thus,  at  the  end  of  the  third  month  the  intes- 
tine contains  a  dark,  greenish,  viscid  material — meconium — composed  of 
bile  pigments,  bile  salts,  and  desquamated  epithelium;  the  amniotic  fluid, 
as  well  as  the  fluid  within  the  bladder,  contains  urea  at  the  end  of  the  sixth 
month,  indicating  the  establishment  of  both  hepatic  and  renal  activity. 
Contractions  of  the  skeletal  muscles  of  the  limbs  begin  about  the  fifth 
month,  from  which  it  may  be  inferred  that  the  mechanism  for  muscle 
activity,  viz.,  muscles,  efferent  nerves,  and  spinal  centers,  has  become 
•  anatomically  developed  and  associated,  and  capable  of  coordinate  activity. 
These  contractions  are,  in  all  probability,  automatic  or  autochthonic  in 
character  due  to  stimuli  arising  with  the  spinal  centers.  The  remaining 
organs  remain  more  or  less  inactive. 

After  birth,  with  the  first  inspiration  and  introduction  of  food  into  the 
alimentary  canal,  the  physiologic  mechanisms  which  subserve  general 
metabolism  begin  to  functionate  and  in  the  course  of  a  week  are  fully 
established.  At  this  time  the  cardiac  pulsation  averages  about  135  a 
minute;  the  respiratory  movements  vary  from  30  to  35  a  minute,  and  are 
diaphragmatic  in  type;  the  urine,  which  was  at  first  scanty,  is  now  abun- 
dant and  proportional  to  the  food  consumed;  the  digestive  glands  are 
elaborating  their  respective  enzymes,  digestion  proceeding  as  in  the  adult. 
The  hepatic  secretion  is  active  and  the  lower  bowel  is  emptied  of  its  con- 
tents; the  coordinate  activities  of  the  nerve-,  muscle-,  and  gland-mechan- 
isms are  entirely  reflex  in  character.  Psychic  activities  are  in  abeyance 
by  reason  of  the  incomplete  development  of  the  cerebral  mechanisms. 


INDEX 


ABDUCENT.NERVE.  209 
Absorption,  86 

by  blood-vessels,  92 

by  lacteals,  92 

of  oxygen  in  respiration,  131 
Accommodation  of  the  eye,  233 
Adrenal  bodies,  166 
Adult   circulation,  establishment  of,  at 

birth,  239 
Air,  atmospheric,  composition  of,  130 

amount  exchanged  in  respiration, 
129 

changes  in,  during  respiration,  131 
Alcohol,  action  of,  s* 
Alimentary  principles,  classification  of, 
55 

carbohydrate  principles,  56. 

fat  principles,  56 

inorganic  principles,  57 

protein  principles,  55 
Amino-acids,  78,  79 
Amnion,  formation  of,  257 
Animal  heat,  136 

Anterior  columns  of  spinal  cord,  173 
Aphasia,  202 
Area,  germinal,  242 
Arteries,  properties  of,  lis 
Articulations,  or  joints,  1 1 

classification  of,  1 1 
Astigmatism,  236 
Autonomic  nerve  system,  205 

functions  of,  208 

BILE,  80 

mode  of  secretion,  82  ] 

physiologic  action,  83 
Bladder,  urinary,  149 
Blastodermic  membranes,  256 
Blood,  94 

changes  in,  during  respiration,  132 

circulation  of,  102 

coagulation  of,  96 

coloring-matter  of,  95 1  98 

composition  of,  plasma,  97 

corpuscles,  98 

gases  of,  132 

origin  of,  99 

pressure,  117 

rapidity  of  flow  in  arteries,  120 

rapidity  of  flow  in  capillaries,  120 
Bronchial  innervation,  12s 

CAPILLARY  BLOOD-VESSELS,  115. 

116 
Capsule,  internal,  189 

external,  190 
Cardiac  cycle,  107 


Caudate  nucleus,  189 
Cells,  structure  of,  5 

manifestations  of  life  by,  7 

reproduction  of,  9 
Center  for  articulate  language,  202 
Central  organs^  of  the  nerve  system  and 

their  nerves,  170 
Cerebellum,  203 

forced  movements,  205 
Cerebrum,  191 

fissures  and  convolutions,  192 

functions  of,  194 

localization  of  functions,  198 

motor  area  of,  200 
speech  area,  202 
writing  area,  202 

sensor  areas  of,  196 
Chorda  tympani  nerve,  course  and  func- 
tion of,  215,  216 
Chorion,  258 
Chyle,  93 

Ciliary  muscle,  228,  233 
Circulation  of  blood,  102 
Claustrum,  189 
Cochlea,  244 

Columns  of  spinal  cord,  173 
Corium,  ISO 

Corpora  quadrigemina,  187 
Corpus  luteum,  253 

striatum,  188 
Corti,  organ  of,  245 
Cranial  nerves,  209 
Crura  cerebri,  186 
Crystalline  lens,  229 

DEGLUTITION,  65 
Digestion,  61 
Ductus  arteriosus,  259 
venosus,  259 

EAR,  238 
Electrotonus,  so 
Embryo,  activities  of,  259 
Embryonic  shield,  256 
Endolymph,  245 
Epidermis,  150 
Epididymis,  254 
Eustachian  tube,  240,  243 
Excretion,  140 
Eyeball,  movements  of,  237 
Eye,  226 

blind  spot  of,  234 

refractmg  apparatus  of,  231 

FACIAL  NERVE,  215 

paralysis,  symptoms  of,  216 
Fallopian  tubes,  251 


261 


262 


INDEX 


Feces,  86 

Fermentation,  intestinal,  8s 

Female  organs  of  generation,  251 

Fetus,  circulation  of  blood  in,  259 

Fissures  and  convolutions  of  brain,  187 

Foods  and  dietetics,  52 

animal,  59 

carbohydrate  principles  of,  55 

cereal,  60 

daily  amount  required,  53 

energy  of,  53.  54 

fat  principles  of,  55 

inorganic  principles  of,  55 

percentage  composition  of,  59 

protein  principles  of,  55 

vegetable,  61 
Fovea  centralis,  228 

GALVANIC    CURRENTS.    EFFECT 

on  nerves,  50 
Ganglia,  40 

Gasserian,  213 

ophthalmic,  206 

semilunar,  207 

spheno-palatine,  215 
Gastric  digestion,  67 

glands,  67 

juice,  69 

action  of,  71 
Generation,  male  organs  of,  254 

female  organs  of,  251 
Globules  of  the  blood,  98 
Glomeruli  of  the  kidneys,  146 
Glosso-pharyngeal  nerve,  217 
Glottis,  respiratory  movements  of,  131 
Glycogen,  160 

Glycogenic  function  of  the  liver,  160 
Goll,  column  of,  173 
Graafian  follicles,  251 

HAIR,  150 

Hearing,  sense  of,  237 

Heart,  102 

auriculoventricular  bundle,  105 

blood  supply,  109 

course  of  blood  through,  103 

ganglia  of,  112 

influence   of   pneumogastric   nerve 
upon, 112 

influence  of  nerve  system  upon,  112 

intraventricular  pressure,  109 

Keith  Flack  node,  106 

sounds  of,  108 

valves  of,  104 
Heat  production,  137 

dissipation,  139 
Hemianopsia,  a  10 
Hemoglobin,  95.  98 
Hyaloid  membrane,  229 
Hypermetropia,  236 
Hypoglossal  nerve,  222 
Hypophysis  cerebri,  165 

INCUS  BONE,  240 
Insalivation,  63 

nerve  mechanism  of,  64 
Inspiration,  movements  of  thorax  in,  127 


Internal  capsule,  189 

results  of  injury  to,  191 

secretion,  162 
Intestinal  digestion,  74 

juice,  physiologic  action,  79 
Intra-pulmonic  pressure,  126 

thoracic  pressure,  126 
Iris,  228 

action  of,  234 

KIDNEYS,  144 

formation  of  urine  by,  147 

LABYRINTH  OP  INTERNAL  EAR, 
243 

function  of  cochlea,  245 

function  of  semicircular  canals,  245 
Larynx,  246 

Laws  of  muscular  contraction,  50 
Lens,  crystal  line,  229 
Levers,  29 
Liver,  158 

conjugation' of  products  of  protein 
putrefaction,  161 

formation  of  urea,  161 

production  of  glycogen,  160 

secretion  of  bile  by,  159 
Localization  of  functions  in  cerebrum, 

196 
Lungs,  124 

changes    in    blood    while    passing 
through,  117 

movements  of,  128 

vital  capacity  of,  130 
Lymph,  90 

corpuscles,  90 
Lymphatic  glands,  89 

vessels,  origin  and  course  of,  88 

MALLEUS  BONE,  240 
Mammary  glands,  i55 

secretion  of  milk  by,  157 
Miistfcation,  62 

muscles  of,  62 

nerve  mechanism  of,  62 
Medulla  oblongata,  184 

functions  of,  185 
Membrana  tympani,  238 
Menstruation,  252 
Mesoderm  and  notocord,  256 
Middle  ear,  238 
]^ilk,  156 

Motor  centers  of  cerebrum,  200 
Muscles,  properties  of,  17 

changes  in,  during  contraction,  19 

special  physiology  of,  28 
Muscle-fiber,  histology  of,  16 
Myopia,  236,  241 

NERVE,  OLFACTORY,  209 
abducent,  212 
acoustic,  217 
cells,  structure  of,  35 
facial,  215 

fibers,  structure  of,  35 

terminations  of,  37 

glosso-pharyngeal,  217 


INDEX 


263 


Nerve,  hypoglossal,  22a 

impulse,   rate   of   transmission,   of 

49 

motor  occuli,  211 

optic,  210 

pneumogastric,  218 

roots,  function  of  ventral  and  dor- 
sal, 44 

spinal  accessory,  220 

tissue,  histology  of,  35 

trigeminal,  213 

trochlearis,  212 

trunks,  structure  of,  38 
Nerves,  afferent,  43 

classification  of,  42 

cranial,  209 

degeneration  of,  45 

development  and  nutrition  of,  41 

efferenjt,  42 

properties  and  functions  of,  46 

relation  of,  to  spinal  cord,  41 

spine,  40,  42 

vaso-motor,  122 
Nerve  tissue,  physiologjy  of,  34 

autonomic,  205 
Neuron,  35 
Nucleus  caudatus,  189 

lenticularis,  189 

OLFACTORY  NERVES,  209 
Ophthalmic  ganglion,  206 
Optic  nerves,  210 

defects,  235 ; 

functions  of,  189 

thalamus,  189 
Organs  of  Corti,  24s 
Ovaries,  169,  251 
Ovum,  251 

discharge    of,     from     the    ovary, 
252 
Oxygen,  absorption  of,  by  hemoglobin, 
99,  114 

PANCREAS,  75 
Pancreatic  juice,  77 

physiologic  action,  78 
Parathyroids,  164 
Peptones,  72 
Perilymph,  244 
Perspiration,  151 
Petrosal  nerves,  large  and  small,  215, 

216 
Phonation,  246 
Pituitary,  165 
Physiology,  definition  of,  2 
Placenta,   formation   and   function   of, 

258 
Pleura,  126 

Pneumogastric,  vagus  nerve,  218 
Pons  varolii,  185 
Portal  vein,  87 
Prehension,  61 
Presbyopia,  23s 

Pressure  in  blood  in  arteries,  117,  119 
Ptyalin,  89 
Pulse,  119 
Pyramidal  tracts,  182 


RED  CORPUSCLES  OP  BLOOD,  98 

chemic  composition,  98 

function,  99 

origin  of,  99 
Reflex  movements  of  spinal  cord,  175 
Reproduction,  251 
Respiration,  123 

chemistry  of,  130 

establishment  after  birth,  135 

movements  of,  127 

nerve  mechanism,  134 

types  of,  128 

total  respiratory  exchange,  153 

volumes  of  air  breathed,  129 
Retina,  228 
Rigor  mortis,  26 


SALIVA,  63 

physiologic  action  of,  64 

Sebaceous  glands,  162 

Semen,  255 

Semicircular  canals,  244,  245 

Sight,  sense  of,  226 

Skeleton,  physiology  of,  9 

Skin,  149 

Smell,  sense  of,  225 

Sounds  of  heart,  109 

Speech,  249 

Spermatozoa,  255 

Spheno-palatine  ganglion,  215 

Spinal  accessory  nerve,  220 
cord,  172 

as  an  independent  center,  173 
function  of,  as  a  conductor,  178 
nerves,  origin  of,  40.  41 
reflex  action  of,  176 
segmentation  of,  172 
structure  of  gray  matter,  172 
special  centers  of,  178 
reflex  excitation,  175 

Starvation,  phenomena  of,  58 

Stomach,  67 

movements  of,  72 
nerve  mechanism  of,  73 

Sudoriparous  glands,  151 

Supra-renal  capsules,  166 


TASTE,  SENSE  OF,  224 

nerve  of,  224 
Teeth,  62 

Tensor  tympani  muscle,  240,  242 
Testicles,  169,  254 
Thoracic  duct,  89 
Thorax,  enlargement  of,  in  inspiration, 

127 
Thyroid  gland,  162 
Tongue,  222 

motor  nerve  of,  222 

sensory  nerve  of,  213 
Touch,  sense  of,  222 
Trachea,  124 
Trochlearis  nerve,  212 


UREA,  141 
Uric  acid,  143 


264 


INDEX 


Urination,  mechanism  of,  149  Vascular  glands,  162 

Urine,  140  Vaso-motor  nerves,  origin  of,  122 

average  quantity  of  solids  secreted  Veins,  116 

daily,  141  Vertebral  column,  10 

composition  of,  141  Vesiculae  seminales,  254 

Uterus,  252  Villi,  structure  and  functions,  92 

Vision,  psychic  center  for,  200 

VAGUS  NERVE,  218  Vital  capacity  of  lungs,  130 

relation  of  to  the  heart,  112  Vocal  bands,  247 

to  respiration,  134  Visual  angle,  232 

Vapor,  water,  of  breath,  13S  Vocal  sounds,  248,  249 


Webster's  Diagnostic  Methods 

CHEMICAL,   BACTERIOLOGICAL  AND   MICROSCOPICAL 

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